DEPARTMENT   OF   COMMERCE 


CIRCULAR 

OP  THE 

BUREAU  OF  STANDARDS 

8.  W.  STRATTON,  DIRECTOR 


No.  113 


STRUCTURE  AND  RELATED  PROPERTIES 
OF  METALS 


[2d  Edition] 
JULY  7,  1922 


PRICK,  25  CENTS 

Sold  only  by  the  Superintendent  of  Documents,  Government  Printing  Office 
Washington.  D.  C. 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1922 


DEPARTMENT   OF   COMMERCE 


CIRCULAR 

OF  THE 

BUREAU  OF  STANDARDS 

S.  W.  STRATTON,  DIRECTOR 


No.  113 


STRUCTURE  AND  RELATED  PROPERTIES 
OF  METALS 


[2d  Edition] 
JULY  7,  1922 


PRICE,   25  CENTS 

Sold  only  by  the  Superintendent  of  Documents,  Government  Printing  Office 
Washington,  D.  C. 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1922 


STRUCTURE  AND  RELATED  PROPERTIES  OF 
METALS  ' 


ABSTRACT 

The  metallographist  uses  the  term  "structure"  to  include  those  structural  features 
which  can  be  revealed  by  examination  either  with  the  unaided  eye  or  by  means  of 
the  microscope.  By  reference  to  numerous  typical  metallographic  examinations 
made  by  the  Bureau  of  Standards  the  general  nature  of  the  structure  of  metals,  the 
methods  for  revealing  it,  and  the  dependence  of  the  properties  of  the  metal  as  a  whole 
upon  its  structural  features  are  discussed. 

Macroscopic  examination  may  reveal  any  or  all  of  the  following:  Chemical  unhomo- 
geneity,  crystalline  heterogeneity,  physical  unsoundness,  and  mechanical  nonuni- 
formity.  The  common  methods  in  use  for  revealing  these  features  are  described 
and  illustrated. 

The  microscopic  examination  necessitates  the  use  of  a  suitable  etching  reagent. 
The  principles  underlying  the  action  of  etching  reagents  are  discussed  and  a  list  given. 
The  principal  conditions  which  affect  structure,  chemical  composition,  application 
of  heat,  and  mechanical  working  of  the  metal  are  discussed  and  illustrated  by  suitable 
examples.  Most  important,  however,  is  the  effect  of  structure  upon  the  properties. 
The  dependence  of  the  mechanical  properties  and  chemical  behavior  upon  the  struc- 
tural condition  of  the  material  is  discussed  at  considerable  length.  The  results  of  a 
large  number  of  "practical  applications"  of  the  study  of  the  structure  of  metals  are 
given  to  show  how  an  explanation  of  the  "failure  in  service"  of  different  types  of 
metals  and  alloys  may  be  reached  by  this  means.  Such  an  explanation  can  often  be 
arrived  at  by  none  of  the  other  testing  methods  in  common  use. 


CONTENTS 

Page 

I .  Purpose  of  circular 4 

II.  Industrial  importance  of  metallography 6 

III .  Methods  for  revealing  the  structure  of  metals 8 

1.  Definition  of  structure 8 

2.  Macroscopic  examination 8 

(a)  Chemical  unhomogeneity  and  crystalline  heterogeneity .  .  9 

(b)  Physical  unsoundness 18 

(c)  Mechanical  nonuniformity 21 

3 .  Microscopic  examination 23 

(a)  Selection  of  typical  specimens 23 

(b)  Preparation  of  specimens 24 

(c)  Methods  of  etching 29 

1  Compiled  by  Henry  S.  Rawdon,  physicist,  from  results  of  metallographic  examinations  made  by  the 
Bureau  of  Standards,  except  where  indicated  otherwise. 


4  Circular  of  the  Bureau  of  Standards 

Page 
IV.  Conditions  affecting  structure 33 

1.  Chemical  composition 33 

2.  Temperature 38 

(a)  Equilibrium  changes 38 

(b)  Grain  growth :|. . ;  .1. 41 

(c)  Phase  changes 43 

3.  Working  of  metals 46 

(a)  Distortion  of  crystalline  structure 46 

V.  Effects  of  structure  upon  properties 48 

1 .  Mechanical  properties 48 

(a)  Hard  and  soft  constituents 48 

(b)  Soft  ductile  constituents 49 

(c)  Orientation  of   test  specimen   with   respect   to  material 

tested 52 

(d)  Coarsely  grained  metals 55 

(e)  Physical  state  of  microscopic  constituents 57 

2.  Chemical  properties 60 

(a)  Etching 60 

(b)  Solubility  of  tempered  steels 61 

(c)  Corrosion 62 

(d)  Variation  in  composition  throughout  an  alloy 66 

VI.  Applications  of  the  microscopy  of  metals 66 

1 .  Relation  to  heat  treatment 67 

2.  Supplement  to  chemical  analysis 73 

3.  Control  of  metallurgical  operations  and  products 82 

4.  Construction  of  constitutional  diagrams 88 

5.  Failure  of  metals  in  service 89 

6.  Service  deterioration  of  alloys 93 

7.  Miscellaneous 97 

VII.  Information  regarding  tests 103 

1.  Reports 103 

2.  Tests 103 

I.  PURPOSE  OF  CIRCULAR 

This  circular  is  not  intended  to  duplicate  or  to  imitate  any  of 
the  excellent  treatises  on  the  subject  of  metallography  which  have 
appeared  in  print  within  the  last  few  years.  It  is  desired  rather, 
by  reference  to  a  series  of  typical  specimens  chosen  from  a  great 
many  which  have  been  submitted  to  this  Bureau  for  examination, 
to  show  the  advantages  of  and  the  necessity  for  the  metallographic 
examination  as  a  means  of  obtaining  complete  and  adequate 
knowledge  of  the  properties  of  metallic  materials.  The  circular 
will  also  aid  very  materially  in  answering  the  numerous  inquiries 
received  by  the  Bureau  concerning  proper  and  suitable  methods 
for  revealing  metal  structures  and  the  interpretation  of  the  results 
obtained.  The  circular  supplements  one  of  earlier  date  2  and,  in 
addition  to  showing  by  means  of  typical  illustrations  the  indus- 
trial importance  of  the  method,  summarizes  different  lines  of 

1  Metallographic  Testing,  B.  S.  Circular  No.  42. 


Structure  and  Properties  of  Metals  5 

metallographic  testing  for  which  the  Bureau  of  Standards  is 
equipped.  A  general  view  of  the  laboratory  devoted  to  the  study 
of  the  structure  of  metals  is  shown  in  Fig.  i . 


The  term  "metallography"  has  come  to  be  used  by  many  in  a 
relatively  narrow  sense  as  synonymous  with  "microscopy  of 
metals."  As  used  by  the  Bureau  of  Standards,  however,  the  term 


6  Circular  of  the  Bureau  of  Standards 

includes  all  those  methods  of  examination  which  throw  some  light 
upon  the  structure  and  properties  of  metals  and  may  properly  be 
considered  as  synonymous  with  "physical  metallurgy."  The 
study  of  the  structure  of  metals  constitutes,  then,  only  one  phase 
of  a  complete  metallographic  examination  of  any  material;  sub- 
sequent publications  similar  to  this  are  planned  to  cover  other 
phases  of  the  subject. 

II.  INDUSTRIAL  IMPORTANCE  OF  METALLOGRAPHY 

It  is  no  longer  necessary  to  offer  arguments  concerning  the 
importance  of  the  metallographic  method.  The  remarkable 
growth  of  metallography  in  the  last  few  years,  together  with  the 
proportionately  widened  scope  of  its  applications,  is  sufficient  and 
ample  evidence  of  the  value  of  the  method.  From  a  relatively 
unimportant  branch  of  physical  chemistry  it  has  been  developed 
into  a  means  of  investigation  of  the  properties  of  metals  and 
alloys  on  a  par  with  the  older  methods  of  chemical  and  mechanical 
testing  and  is  very  frequently  of  service  in  explaining  difficulties 
which  are  inexplicable  by  the  other  and  older  methods  alone. 

It  is  in  the  metallurgy  of  iron  and  steel  that  the  metallographic 
method  has  found  widest  application.  This  is  only  a  natural 
consequence  of  the  great  industrial  importance  of  steel  as  well  as 
of  the  complex  nature  of  the  alloy  itself.  For  the  purpose  of  sup- 
plementing and  interpreting  the  results  of  chemical  analysis  of 
such  metallic  products  the  method  is  of  great  importance.  By 
means  of  preliminary  examinations  of  specimens  to  be  sampled, 
the  unhomogeneity  of  the  material  may  be  determined  so  that  a 
sample  may  be  chosen  in  such  manner  that  it  will  properly  rep- 
resent the  material.  On  the  other  hand,  at  times  it  is  desirable 
to  choose  the  sample  so  that  it  will  demonstrate  in  what  manner 
the  composition  of  the  special  material  differs  from  the  normal. 
Likewise  the  knowledge  gained  from  the  examination  will  aid  in 
explaining  apparent  discrepancies  obtained  in  the  analysis  of 
samples  taken  from  different  parts  of  the  specimen,  or  from  sup- 
posedly similar  specimens. 

The  physical  properties  of  an  alloy  are  much  more  closely 
related  to  the  minute  structure  of  the  material  than  they  are  to 
the  ultimate  chemical  composition.  To-day  no  one  questions  the 
value  of  chemical  analysis  in  metallurgical  work,  but  the  metal- 
lographic examination  when  properly  interpreted  may  be  of  far 
greater  value  than  the  chemical — for  example,  in  explaining  the 


Structure  and  Properties  of  Metals  7 

properties  and  predicting  the  uses  of  the  finished  product.  The 
nature  of  the  various  microconstituents  comprising  the  alloy, 
their  relative  size  and  distribution,  the  occurrence  of  extraneous 
substances  or  "inclusions,"  the  structural  effects  of  thermal  and 
mechanical  treatment,  together  with  other  features  revealed  by 
the  examination,  are  factors  of  supreme  importance  in  determin- 
ing the  properties  of  the  material.  By  way  of  illustration,  a 
medium  carbon  steel  may  be  mentioned;  this  alloy  may  have 
mechanical  properties  ranging  at  one  end  from  those  of  a  very 
hard  metal  of  no  appreciable  ductility  and  almost  impossible  to 
machine,  to  one  at  the  other  extreme,  fairly  soft  and  ductile  and 
readily  machined.  The  composition  is  constant  throughout  and 
tells  us  nothing.  For  the  proper  explanation  a  knowledge  of  the 
structural  condition  which  has  been  brought  about  by  the  various 
treatments  to  which  the  metal  has  been  subjected  is  necessary 
and  usually  sufficient. 

All  tests  of  metals  are  for  the  general  purpose  of  determining 
the  suitability  of  the  material  for  some  specific  use ;  the  mechanical 
properties  are  therefore  in  many  cases  the  ultimate  criteria.  As 
in  the  case  of  chemical  analysis  the  metallographic  examination 
can  be  made  of  inestimable  service  in  mechanical  testing.  The 
selection  of  specimens  may  determine  absolutely  the  validity 
of  any  conclusions  drawn  from  tests.  Not  only  may  the  small 
test  specimens  be  properly  taken  so  as  to  represent  the  larger 
mass  of  metal,  but  the  results  obtained  in  the  test  may  be  most 
properly  interpreted  in  terms  of  the  structure  of  the  material,  and 
for  purposes  of  comparison  it  is  necessary  that  the  materials  be 
in  the  same  structural  condition.  In  the  following  discussion 
these  features  are  treated  more  fully. 

A  knowledge  of  the  chemical  composition  is  essential  to  a  full 
understanding  and  interpretation  of  the  structural  condition  of 
any  metal  or  alloy,  and  to  a  somewhat  lesser  degree  mechanical 
testing  is  also.  Conversely,  a  knowledge  of  the  structural  condi- 
tion supplements  and  explains  the  results  of  the  other  two  methods 
of  testing.  All  three  are  mutually  interdependent  and  necessary 
for  a  full  understanding  of  the  properties  of  any  metal. 


8  Circular  of  the  Bureau  of  Standards 

III.  METHODS  FOR  REVEALING  THE  STRUCTURE  OF 
METALS 

1.  DEFINITION  OF  STRUCTURE 

The  term  "structure"  when  employed  with  reference  to  metals 
and  alloys  is  used  in  a  somewhat  restricted  sense.  The  metal 
microscopist  ordinarily  does  not  include  in  his  definition  of  this 
term  such  characteristics  as  the  minute  crystalline  structure,  the 
arrangement  of  atoms,  and  such  other  fundamental  features  of  the 
structure  of  matter  as  may  be  revealed  by  suitably  refined  means. 
The  term  is  used  to  include  those  features  for  revealing  which  no 
refinement  greater  than  that  of  the  modern  compound  microscope 
is  necessary.  It  should  be  borne  in  mind,  however,  that  in  a  few 
special  cases  recourse  must  be  had  to  very  special  means  for  suit- 
ably revealing  the  structural  features  of  the  metal. 

2.  MACROSCOPIC  EXAMINATION 

The  study  of  the  structure  of  any  metal  most  properly  begins 
with  the  macroscopic  examination  of  the  specimen;  that  is,  with 
an  examination  which  does  not  involve  any  magnification  other 
than  that  obtained  by  the  use  of  the  simple  magnifier.  It  is  quite 
evident  that  the  knowledge  of  the  gross  structure  of  alloys  and 
metals  gained  by  the  preliminary  macroscopic  examination  is 
very  helpful  in  understanding  properly  the  more  minute  features 
revealed  by  the  microscope  in  exactly  the  same  way  that  a  knowl- 
edge of  the  anatomy  of  the  human  body  must  be  used  as  a  back- 
ground in  which  to  fit  the  information  gained  by  a  study  of  the 
histological  or  minute  features  of  the  various  tissues  which  make 
up  the  body.  Although  this  survey  is  usually  made  for  the 
purpose  of  revealing  chemical  unhomogeneity,  generally  by  some 
suitable  etching  method,  other  important  structural  features  are 
often  revealed.  Some  of  these  are:  Crystalline  heterogeneity — 
for  example,  relative  size,  shape,  and  arrangement  of  crystal 
grains,  lack  of  grain  refinement,  persistence  of  "casting  struc- 
ture" after  heat  treatment  and  mechanical  working;  physical 
unsoundness  such  as  "flakes"  and  internal  fractures,  blowholes, 
gas  cavities,  and  porosity ;  variations  in  structure  due  to  heat  treat- 
ment incidental  to  such  processes  as  welding  and  oxyacetylene 
cutting  of  metals;  local  deformation  caused  by  such  processes  as 
riveting  and  punching;  and  other  structural  features  that  may 
occasionally  be  met.  By  special  methods  information  relating 


Structure  and  Properties  of  Metals  9 

to  the  mechanical  state  of  the  metal — that  is,  the  distribution  and 
relative  magnitude  of  internal  stresses — may  also  be  gained. 
The  different  methods  in  use  for  the  study  of  the  macrostructure 
of  metals  may  be  best  described  in  connection  with  the  different 
purposes  for  whigh  macroscopic  examinations  are  made.  In  the 
following  discussion  the  applications  for  the  study  of  the  ferrous 
alloys  will  be  described  in  much  greater  detail  than  for  the  non- 
ferrous  ones,  inasmuch  as  the  method  has  been  developed  for  the 
study  of  iron  and  steel  to  a  higher  degree  than  for  other  metals. 

(a)  CHEMICAL  UNHOMOGENEITY  AND  CRYSTALLINE  HETEROGENEITY 

One  of  the  most  serious  and  most  common  of  the  defects  of 
alloys  revealed  by  macroscopic  examination  is  the  lack  of  chemical 
homogeneity.  Such  variations  in  composition  may  be  brought 
about  in  the  material  intentionally  as,  for  example,  in  case- 
hardened  steels,  partially  malleableized  cast  iron,  and  similar 
products,  in  which  case  it  is  hardly  to  be  classed  as  a  defect.  In 
the  greater  number  of  examples  by  far,  however,  chemical  unhomo- 
geneity  represents  an  undesirable  state  resulting  from  conditions 
of  manufacture,  such  as  segregation  and  liquation.  Such  con- 
ditions often  are  so  pronounced  that  they  persist  in  the  metal 
throughout  the  different  treatments,  both  mechanical  and  thermal, 
that  it  receives,  and  so  appear  in  the  metal  in  its  finished  state. 
They  may  thus  serve  the  useful  purpose  of  furnishing  a  record  of 
the  plastic  flow  of  the  metal  during  the  various  manufacturing 
operations,  as  will  be  referred  to  later.  Variations  in  the  chemical 
composition  of  any  alloy  quite  generally  result  in  a  differential 
attack  of  the  material  when  it  is  subjected  to  the  action  of  a 
corrosive  or  etching  reagent  of  any  kind.  The  greater  the  dif- 
ferences in  composition  in  different  portions  of  the  metal,  the  more 
pronounced  are  the  relative  differences  in  the  etch  pattern  which 
results. 

The  most  commonly  used  etching  reagent  for  steels  for  macro- 
scopic examinations  is  an  aqueous  solution  of  copper-ammonium 
chloride  (Heyn's  reagent).  The  solution  usually  recommended  is 
approximately  8  per  cent  in  strength,  10  g  in  120  cm3  of  water. 
The  solution  keeps  indefinitely  and,  if  desired,  may  be  diluted 
somewhat  when  used.  The  experience  of  the  Bureau  of  Stand- 
ards indicates  that  a  somewhat  weaker  solution  than  the  above, 
the  specimen  being  etched  two  or  three  times  in  a  fresh  solution, 
if  necessary,  is  the  most  convenient  way  to  use  this  reagent. 
The  specimen,  after  it  has  been  sectioned  and  roughly  polished, 


ro  Circular  of  the  Bureau  of  Standards 

carefully  cleaned  free  from  oil  marks  or  finger  prints  by  washing 
in  alcohol  or  gasoline  and  then  dried,  is  immersed  in  the  solution, 
polished  surface  up.  Care  must  be  taken  so  that  the  solution 
quickly  covers  the  entire  surface,  that  no  air  bubbles  are  en- 
trapped, and  that  the  liquid  is  agitated  gently;  otherwise  queer, 
misleading  markings  on  the  etched  surface  may  result.  If  desired 
the  surface  of  the  metal  may  be  rubbed  with  a  little  emery  flour 
on  the  tip  of  the  finger,  washed  with  water,  and  immersed  while 
still  wet  so  as  to  promote  the  even  flow  of  the  etching  reagent 
over  it.  A  coating  of  spongy  copper  forms  over  the  face  of  the 
specimen;  this  is  easily  removed,  however,  with  a  swab  of  wet 
cotton,  if  the  etching  solution  was  of  the  proper  concentration, 
and  the  portions  of  high  carbon,  sulphur,  and  phosphorus  content 
will  be  found  to  have  been  darkened  as  a  result  of  the  etching. 
If  the  copper  film  adheres  and  can  not  be  removed,  because  of 
improper  concentration  or  temperature  of  the  solution,  a  dilute 
aqueous  solution  (approximately  0.5  per  cent)  of  ammonium 
persulphate  will  facilitate  in  its  removal. 

Results  somewhat  similar  to  those  of  Heyn's  reagent  may  be 
obtained  by  the  use  of  a  solution  of  iodine  (10  g  iodine,  20  g 
potassium  iodide,  and  100  cm3  water).  The  etch  markings  are 
not  so  clearly  defined,  however,  as  in  the  case  of  the  copper- 
ammonium  chloride  solution.  The  iodine  solution  was  formerly 
used  much  more  than  it  is  at  present. 

Concentrated  acid  may  be  employed  to  advantage  to  reveal 
chemical  unhomogeneity.  Hot  (100°  C)  concentrated  hydro- 
chloric acid  is  often  used,  although  others  are  sometimes  pre- 
ferred by  different  workers.  Such  other  acids  used  include  i-i 
nitric  acid,  various  concentrations  of  hydrochloric  acid,  dilute 
sulphuric  acid  (20  cm3  concentrated  acid,  100  cm3  water),  and 
various  other  mixtures.  The  general  result  is  the  same  in  all 
cases.  The  highly  contaminated  portions  are  etched  out,  and  the 
surface  is  roughened  considerably.  The  etch  pattern  produced 
by  a  prolonged  attack  by  a  dilute  acid  is  often  much  less  sharp 
and  distinct  than  one  produced  in  the  same  material  by  a  rapid 
attack  by  a  concentrated  acid.  A  prolonged  etching — for  exam- 
ple, 4  or  5  hours — in  5  per  cent  alcoholic  solution  of  picric  acid 
is  often  very  useful,  however.  The  deeply  etched  specimen  may 
be  used  for  producing  a  permanent  record  by  inking  the  face  with 
printer's  ink  and  making  a  print  on  paper.  Fig.  2,  which  shows 
the  head  of  a  rail  submitted  to  the  Bureau  of  Standards  for 


Structure  and  Properties  of  Metals 


ii 


FIG.  2. — Macrostructure  of  the  head  of  a  segregated  rail  -which  failed  in  service,  as  revealed 
by  various  etching  reagents.     X  3/4 

Note  that  some  of  the  reagents  are  much  more  effective  than  others  in  revealing  the  segregated  center  of 
the  head.  Although  such  metal  should  be  regarded  with  suspicion,  it  is  not  to  be  inferred  that  segregation 
of  the  character  shown  will  of  necessity  lead  to  failure  of  the  rail.  The  character  of  the  service  is  the  deci- 
sive factor.  Methods  of  etching :  (a)  Sulphur  print,  (6)  aqueous  solution  of  copper  ammonium  chloride 
(Heyn's  reagent),  (c)  aqueous  solution  of  iodine  and  potassium  iodide,  (rf)  acidulated  alcoholic  solution  of 
cupric  chloride  (Stead's  reagent),  (e$  hot  concentrated  hydrochloric  acid,  (/)  print  of  the  deeply  etched 
surface  of  e 


12  Circular  of  the  Bureau  of  Standards 

examination  after  failure  in  service  in  the  track,  illustrates  the 
use  of  the  various  reagents  described  above.  It  is  very  evident 
that  the  results  of  chemical  analyses  of  the  rail  shown  by  several 
analysts  might  differ  very  materially  according  to  the  location  in 
the  rail  of  the  sample  used  for  the  analysis. 

Fig.  3  shows  the  appearance  of  a  specimen  deeply  etched  with 
concentrated  hydrochloric  acid.  The  specimen  was  a  portion  of 
a  large  steel  casting  which  broke  during  shipment.  Although  the 


FIG.  3. — Macrostructure  of  a  defective  steel  casting  revealed  by  deeply  etching  with  hot 
,    concentrated  hydrochloric  acid.     X  1/2 

Note  the  fern-like  pattern  which,  in  steel,  is  indicative  of  inferior  material 

material  had  been  given  the  usual  specified  annealing  for  grain 
refinement,  the  metal  was  so  porous  and  segregated  in  character 
that  the  original  structural  pattern,  and  also  the  accompanying 
inferior  mechanical  properties,  were  largely  retained  and  not 
materially  improved  by  the  annealing. 

A  reagent  of  decided  merit  for  revealing  the  macroscopic  fea- 
tures of  iron  and  steel  is  an  aqueous  solution  of  ammonium  per- 
sulphate ( i  or  2  g  in  10  cm3  water) .  This  has  long  been  recognized 


Structure  and  Properties  of  Metals  13 

as  one  of  the  best  reagents  for  etching  copper  alloys,  but  its  appli- 
cation  to   the   ferrous   alloys   has   been   neglected.3     It   reveals 


FlG.  4. — Macrostructure  of  fusion  welds  and  of  segregated  steel,  illustrating  the  advan- 
tages of  ammonium  persulphate  as  an  etching  reagent.     X  i 

A  coarsely  crystalline  condition  is  usually  considered  very  undesirable  in  metals.  Note  that  this  con- 
dition is  revealed  most  plainly  by  ammonium  persulphate,  (a)  Specimen  of  oxyacetylene  welded  steel 
plate  etched  with  aqueous  solution  of  ammonium  persulphate;  (6)  specimen  a  etched  with  aqueous 
solution  of  copper  ammonium  chloride;  (c)  specimen  a  etched  with  2  per  cent  alcoholic  nitric  acid;  (d) 
welded  steel  plate,  similar  to  a,  illustrating  overheating  of  the  plate,  etched  with  ammonium  persulphate; 
(e)  cross  section  of  bar  of  segregated  steel,  etched  with  ammonium  persulphate 

chemical  unhomogeneity  as  well  as  do  the  reagents  mentioned 
above  and  has  the  added  advantage  in  that  it  shows  crystalline 

3  Henry  S.  Rawdon,  The  Use  of  Ammonium  Persulphate  for  Revealing  the  Macrostructure  of  Iron  and 
Steel,  B.  vS.  Sci.  Papers,  No.  402. 


14  Circular  of  the  Bureau  of  Standards 

heterogeneity  in  a  very  striking  manner.  It  is  probably  the  best 
reagent  for  iron  and  steel  for  showing  this  phase  of  the  structure. 
Fig.  4  illustrates  results  obtained  by  its  use. 

In  addition  to  the  above  reagents  a  number  of  others  are  used, 
some  of  which  are  intended  for  special  purposes.  A  reagent 
described  for  demonstrating  the  distribution  of  sulphur  and 
phosphorus  in  steel  is  an  acidified  aqueous  solution  of  mercuric 
chloride  (10  g  mercuric  chloride;  20  cm3  hydrochloric  acid,  1.12 
specific  gravity;  water  100  cm3).  When  the  specimen  is  immersed 
a  black  precipitate  forms  on  the  areas  of  high  sulphur  content, 
while  yellow  specks  indicate  the  higher  phosphorus  areas.  The 
reagent  is  usually  applied  on  thin  silk,  which  is  pressed  firmly 
against  the  face  of  the  specimen  and  a  permanent  print  is  thus 
made.  The  reagent  is  but  little  used  in  this  country  because  the 
same  information  may  be  obtained  more  easily  with  other  reagents. 

The  distribution  of  sulphur  is  usually  shown  by  the  so-called 
sulphur-print  method.  The  steel  specimen  is  sectioned,  and  the 
surface,  after  being  smoothed  off  with  a  file,  is  pressed  firmly 
against  a  sheet  of  photographic  paper  which  has  been  moistened 
with  a  dilute  sulphuric  acid  solution.  Two  cm3  concentrated 
acid  in  100  cm3  water  is  the  concentration  generally  used,  though 
in  many  cases  a  more  dilute  one  may  be  used  to  advantage. 
Particularly  is  this  the  case  with  metal  very  high  in  sulphur  and 
also  for  very  large  specimens  where  considerable  time  is  needed 
for  placing  the  paper  in  position.  Mat-finish  paper  should  be 
used.  It  is  extremely  difficult  to  prevent  glossy  paper  from 
slipping  on  the  metal  surface,  in  which  case  a  blurred  print 
results.  A  fine  polish  of  the  surface,  such  as  is  essential  for 
microscopic  examination,  is  not  necessary  nor  desirable  for  sul- 
phur printing.  Very  clear  prints  can  be  made  on  surfaces 
finished  with  a  medium  fine  file.  Bromide  paper  or  any  of 
the  common  developing  photographic  papers  may  be  used. 
The  work  of  LeChatelier  and  Bogitch 4  indicates  that  the 
darkening  of  the  sensitized  surface  of  the  photographic  paper 
is  caused  by  the  action  of  the  acid  upon  the  sulphur  alone  and 
not  by  the  combined  action  of  sulphur  and  phosphorus,  as  has 
formerly  generally  been  supposed.  The  assumption  is  always 
made,  however,  that  any  unhomogeneity  which  may  exist  for  the 
metalloids  other  than  sulphur  occurs  under  the  same  conditions 
as  does  that  of  sulphur,  and  that  the  distribution  of  sulphur 

«  H.  I<eChatelier  and  B.  Bogitch,   "Macrographie  des  aciers."  Rev.  de  Metallurgie,  Memoirs  16,  p.  I3Si. 
1919. 


Structure  and  Properties  of  Metals  15 

recorded  in  the  sulphur  print  is  an  index  of  the  distribution  of  the 
other  constituents  of  the  steel  also.  Fig.  2  shows  this  similarity 
of  sulphur  print  to  the  etch  patterns  obtained  by  the  usual  etching 
methods.  The  length  of  time  required  for  printing  depends 
upon  the  strength  of  the  acid  solution  used  and  the  sulphur 
content  of  the  metal.  When  steels  high  in  sulphur  are  used, 
several  prints  may  be  made  without  regrinding  the  surface,  by 
progressively  increasing  the  printing  time  considerably  for  prints 
after  the  first  one.  By  the  use  of  a  suitable  press,  prints  which 
are  as  clear  and  definite  as  the  photographs  of  the  etched  surfaces 
may  be  obtained. 

By  means  of  a  specially  prepared  gelatinous  emulsion  of  silver 
bromide  the  sulphur-print  method  may  be  extended  to  the  study 
of  fractures.  Directions  for  the  preparation  of  the  emulsion  have 
been  published.5'6  This  special  application  of  the  method  is  but 
little  used,  however. 

An  acidulated  alcoholic  solution  of  copper  chloride  is  often  used 
to  reveal  the  distribution  of  phosphorus.  Various  formulas  for 
preparing  this  reagent  have  been  recommended  by  different 
investigators.  In  general,  however,  the  results  obtained  are 
very  similar  for  all  of  them.  The  formula  of  Stead  (copper 
chloride,  10  g;  magnesium  chloride,  40  g;  concentrated  hydro- 
chloric acid,  20  cm3;  and  alcohol  to  make  1000  cm3)  and 
that  of  LeChatelier  (95  per  cent  alcohol,  100  cm3;  water,  10  cm3; 
copper  chloride,  i  g;  picric  acid,  0.5  g;  and  concentrated  hydro- 
chloric acid,  from  i  to  3  cm3)  may  be  cited  as  representative. 
After  an  immersion  of  about  one  minute,  the  previously  polished 
and  cleaned  surface  of  the  specimen  will  usually  be  found  to  be 
covered  in  certain  portions  with  a  firmly  adhering  film  of  copper. 
Other  parts  remain  clear  or  uncoated.  The  copper-coated 
portions  are  generally  considered  to  be  of  lower  phosphorus  con- 
tent than  the  clear  or  slightly  coated  portions.  Other  elements 
in  solid  solution,  however,  such  as  nickel,  may  produce  a  similar 
differential  precipitation  of  the  copper,  and  the  work  of  LeChate- 
lier and  Bogitch  7  appears  to  indicate  that  the  reagent  is  pri- 
marily useful  for  demonstrating  the  distribution  of  oxide  within 
the  steel,  the  other  indications  being  only  secondary  ones.  Fig.  5 
shows  a  portion  of  a  shrapnel  shell  which  was  etched  with  this 
reagent. 

5  F.  Rogers,  "The  investigation  of  fractures,"  J.  Iron  and  Steel  Inst.,85,  p.  379;  1912. 
8  A.  Portevin,  "  Les  cassures  defectueuses,"  Rev.  de  Metallurgie,  Memoirs,  16,  p.  340;  1919. 
7  H.  I^eChatelier  and  B.  Bogitch,  "Macrographie  des  aciers,"  Rev.  de  Metallurgie,  Memoirs,  16,  p.  129; 
1919. 


1 6  Circular  of  the  Bureau  of  Standards 

The  cupric  reagents  have  largely  supplanted  the  method  of  heat 
tinting  for  showing  the  distribution  of  phosphorus.  However, 
this  latter  method  is  very  useful  for  cast  iron,  particularly  for 
microscopic  examination,  as  will  be  referred  to  later. 

For  revealing  the  macrostructural  features  of  brasses,  bronzes, 
and  similar  alloys  of  high  copper  content,  an  aqueous  solution  of 
ammonium  persulphate  is  very  often  employed,  although  an 
acidified  solution  of  ferric  chloride  or  an  ammoniacal  solution  of 
copper-ammonium  chloride  may  be  used  with  very  good  results. 
The  relative  size  and  arrangement  of  the  crystals  are  the  features 
in  which  one  is  most  interested ;  that  is,  in  addition  to  the  matter 
of  soundness  in  the  case  of  castings.  In  this  case,  however,  an 


FIG.  5. — Macrostructure  of  forged  steel  (longitudinal   section  of  the  head  of  a  shrapnel 
shell)  revealed  by  etching  with  cupric  chloride  (Stead's  reagent).     X  4/5 

The  "banded"  structure  indicates  a  nonuniform  distribution  of  phosphorus  in  the  steel  and  is  con- 
ducive to  brittleness  particularly  when  the  material  is  stressed  transversely  to  such  streaks 

etching  is  usually  unnecessary.     In  Fig.  6  specimens  are  shown 
to  illustrate  the  usual  macroscopic  features  of  copper  alloys. 

The  reagent  commonly  employed  for  etching  aluminum  and 
its  alloys  to  reveal  the  macrostructure  is  an  aqueous  solution  of 
sodium  hydroxide  (approximately  10  per  cent).  It  is  customary 
to  heat  the  specimen  in  the  solution  until  the  surface  is  sufficiently 
etched.  A  combination  of  alkali  and  hydrofluoric  acid  etching 
has  been  highly  recommended  by  Carpenter.8  The  specimen  is 

8  H.  C.  H.  Carpenter  and  C.  F.  Elam,  "Crystal  growth  and  recrystallization  in  metals,"  J.  Inst.  of  Metals 
4;   1920. 


Structure  and  Properties  oj  Metals  r 

aim  iiiiifMni ii'iirnnnMif '!> " 


pIG.  b— Micro  structure  of  a  forged  copper  alloy,  manganese  bronze,  revealed  by  etching 
with  an  ammoniacal  solution  of  copper  ammonium  chloride 

Note  the  cracks  which  occurred  in  service  in  the  coarsely  crystalline  specimens,  a  and  6;  presumably  this 
was  largely  the  result  of  the  crystalline  condition  of  the  material,     (a)  Longitudinal  section  of  the  head  of 
a  forged  bolt  which  cracked  in  service  (X  i'A) ;  (b)  longitudinal  section  of  bolt  which  failed  in  service  (X  3) ; 
(c)  longitudinal  section  of  bolt  (X  2) 
110580°— 22 2 


i8 


Circular  of  the  Bureau  of  Standards 


etched  in  an  alcoholic  solution  of  sodium  hydroxide  to  remove  the 
"flowed"  surface  metal.  When  the  surface  has  been  faintly 
etched,  the  sample  is  transferred  to  a  dilute  aqueous  hydrofluoric 
acid  solution  (i  or  2  per  cent)  and  allowed  to  remain  until  the 
etching  is  complete. 

Fig.  7  shows  specimens  of  an  aluminum  alloy,  designated  as 
"conducting  aluminum"  and  of  the  approximate  composition — 
silicon,  0.5  per  cent;  iron,  0.5  per  cent;  magnesium,  0.7  per  cent; 
aluminum,  remainder — etched  so  as  to  reveal  the  macrostructure. 
The  occurrence  of  minute  pores  of  intercrystalline  cavities  in 
castings  of  light  aluminum  alloys  is  a  matter  of  grave  importance. 
^_^__  Their  presence  can  often  be 

'~?JI  detected  in  suspected  metal  by 

immersing  a  polished  specimen 
in  alcohol  colored  with  picric 
acid  or  some  other  brightly 
colored  dye.  The  specimen, 
after  rapid  washing  to  remove 
all  traces  of  the  dye  on  the 
surface,  is  dried  and  allowed 
to  stand  in  a  warm  place.  The 
porosity  of  the  metal  is  often 
indicated  by  the  appearance 
of  colored  spots  on  the  surface 
as  the  colored  alcohol  evapo- 
rates from  within  the  internal 
cavities  in  which  it  was  inclosed . 


& 


FIG.  7. — Macrostructure  of  a  wrought  alumi- 
num alloy.     X  2 


Etching  reagent, 
sodium  hydroxide 


(b)  PHYSICAL  UNSOUNDNESS 


Much  of  the  evidence  of  this  phase  of  the  structure  of  metals 
is  furnished  by  a  simple  visual  examination.  In  case  the  unsound- 
ness  is  of  a  minute  character  the  method  described  above  for 
aluminum  may  be  used.  In  other  cases  very  special  means  must 
be  used.  If  the  surface  of  the  specimen  is  carefully  machined, 
a  very  light  finishing  cut  with  a  very  sharp  tool  being  made, 
evidence  as  to  the  true  state  of  the  soundness  of  material  is  often 
made  available  which  can  be  obtained  in  almost  no  other  way. 
The  ordinary  methods  which  involve  polishing  and  etching  exag- 
gerate the  features  of  unsoundness  to  a  considerable  degree. 
In  many  alloys  and  metals  which  are  rather  coarsely  crystalline 
careful  machining  is  often  sufficient  also  to  reveal  the  structure 
of  the  material  to  a  ^ery  surprising  degree. 


Structure  and  Properties  of  Metals  19 

An  examination  by  means  of  X  rays  is  often  of  value  if  the 
specimen  is  not  too  large.  The  features  revealed  by  this  method 
of  examination  are  primarily  those  which  result  from  considerable 
differences  in  density,  hence  it  is  of  great  value  in  locating  internal 
cavities  and  similar  flaws.  Ordinary  segregation  can  not  be 
revealed  by  this  means,  although  it  has  been  successfully  used  in 
detecting  such  features  of  composition  as  resulted  from  the 
addition  of  a  lead  alloy  for  filling  cavities  in  light  aluminum 
castings.  Specimens  of  steel  to  be  examined  by  this  means  should 
not  exceed  one-half  inch  in  thickness.  As  a  general  rule,  the 
thicker  the  specimen  the  more  pronounced  must  be  the  defect 
in  order  that  its  presence  can  be  revealed  by  this  means.  This 
Bureau  has  found  radiographic  examination  most  useful  in  fol- 
lowing the  effect  of  a  series  of  successive  treatments,  thermal  and 
mechanical,  upon  certain  internal  defects  such  as  are  shown  in 
Fig.  8.  The  flaws  revealed  by  the  radiographic  examination  are 
of  the  same  character  as  those  described  later  (Sec.  V,  i,  c). 
It  is  evident  from  Fig.  8  that  they  have  not  been  eliminated  by 
the  treatments  to  which  the  steel  was  subjected.  On  the  other 
hand,  they  appeared  to  have  been  accentuated.  This  method 
has  also  been  used  to  advantage  in  the  examination  of  test  bars 
preliminary  to  carrying  out  a  test,  particularly  such  tests  as  are 
very  time  consuming,  as  fatigue,  or  such  tests  as  would  be  in- 
fluenced greatly  in  their  results  by  internal  defects,  for  example, 
impact.9 

For  detection  of  cracks,  such  as  may  be  produced  by 
hardening  by  quenching  and  similar  operations  upon  steel,  a 
magnetic  method  will  be  found  very  useful.  The  method  is  par- 
ticularly valuable  for  revealing  them  in  an  early  stage  in  the 
shaping  of  an  article — for  example,  precision  gages  and  similar 
pieces  which  must  be  ground  to  size — so  that  defective  specimens 
may  be  discarded  without  much  expenditure  of  wasted  effort. 
The  roughly  polished  specimen  is  magnetized  and  then  immersed 
in  a  light  oil  containing  very  fine  iron  dust  in  suspension.  Kero- 
sene and  "cast-iron  mud,"  such  as  is  obtained  from  lapping  disks, 
may  be  used.  The  iron  particles  bridge  across  any  slight  dis- 
continuity in  the  surface  of  the  specimen  and  locate  very  accurately 
the  system  of  surface  cracks.  The  excess  of  iron  dust  may  be 
removed  by  bathing  the  specimen  in  alcohol  or  clean  kerosene. 
The  method  has  also  been  successfully  used  for  the  detection  of 

9  Henry  S  Rawdon,  "Some  applications  of  metal  radiography,"  Proc.  Am.  Iron  and  Steel  Inst.;  Octo- 
ber, 1919. 


20  Circular  of  the  Bureau  of  Standards 


FIG.  8. — Defects,  "flakes,"  in  forged  gun  steel  revealed  by  radiographic  examination. 

Approximately  X  i 

A  radiograph  of  the  specimen  was  taken  after  each  of  the  treatments  listed  below.  The  steel  plate  Jf 
inch  thick,  containing  three  holes  (white  spots  in  radiographs)  drilled  partly  through  for  reference  points, 
was  placed  so  that  the  direction  of  the  X-rays  coincided  with  the  plane  of  the  defect.  Each  white  line  in 
the  radiograph  represents  a  "  flake"  or  defect.  Treatments :  (a)  Forging,  as  received,  (6)  specimen  o  after 
annealing  30  minutes  at  900°  C.  furnace  cooled,  (c)  specimen  6  heated  30  minutes  at  900°  C  and  quenched 
in  oil,  (d)  specimen  c  heated  30  minutes  at  1050°  C  and  quenched  in  oil.  The  successive  radiographs  indi- 
cate the  persistence  of  the  defects  after  the  thermal  treatments  given  the  material.  Exposure,  9-inch 
spark,  i  milliamperes,  7  minutes 


Structure  and  Properties  of  Metals 


21 


internal  fractures  in  wrotight-steel  parts.10     Fig.  9  shows  specimens 
which  have  been  treated  in  this  manner. 

(c)  MECHANICAL  NONUNIFORMITY 

The  examination  of  metals  for  mechanical  nonuniformity — 
that  is,  for  the  presence  of  internal  stresses  of  high  magnitude 
which  may  later  lead  to  serious  deterioration — may  be  noted  here. 


C 

FIG.  9. — Physical  unsoundness  of  steel  revealed  by  magnetic  examination 

(a)  Roughly  polished  surface  of  a  precision  gage  block.  X  SA.  (b)  Same  as  a,  magnetized  and  bathed 
in  kerosene  containing  fine  iron  dust  in  suspension;  note  the  network  of  fine  "hardening"  cracks  revealed. 
X  iJ4.  (c)  Section  of  steel  from  head  of  a  rail  containing  internal  fracures  revealed  by  method  of  b.  X  3. 
(<0  Same  specimen  as  c  after  locating  the  defect  by  a  punch  mark  at  each  end.  After  removing  the  iron 
dust  no  trace  of  the  discontinuity  could  be  seen.  X  3.  (e)  Same  specimen  as  d,  retreated  as  given 
in  6.  X3 

Fig.  10  a  shows  a  portion  of  a  cold- worked  rod  of  manganese 
bronze  which  has  been  immersed  in  a  solution  of  mercurous  nitrate 
acidulated  with  nitric  acid.  The  cracks  which  were  formed  by 
the  action  of  this  reagent  may  be  taken  as  an  indication  of  what 
would  undoubtedly  have  occurred  spontaneously  later  in  service.11 

10  H.  S.  Rawdon  and  S.  Epstein,  Metallographic   Features  Revealed  by  the  Deep  Etching  of  Steel, 
B.  S.  Tech.  Papers,  No.  156. 

11  H.  S.  Rawdon,  "  The  use  of  mercury  solutions  for  predicting  the  season  cracking  of  brass,"  Froc. 
Am.  Soc.  for  Testing  Materials,  17,  part  2,  p.  189;  1917. 


22 


Circular  of  the  Bureau  of  Standards 


Materials  wliich  crack  readily  when  treated  in  this  manner  can  be 
shown  by  other  means  to  be  highly  stressed  internally.12 

Steel  parts  such  as  balls  and  roller  bearings  which  are  very 
vigorously  hardened  indicate  by  their  behavior  upon  etching  in 
the  proper  manner  that  a  similar  condition  obtains  there.  Fig. 
10  b  shows  several  hardened  steel  balls  which  split  when  they 
were  etched  with  hot  concentrated  acid ;  cold-rolled  steel  shafting 
will  sometimes  behave  similarly  when  etched  in  the  same  manner. 


FIG.   10. — Mechanical  nonuniformity  of  wrought  metals  revealed  by  deep  etching 

Cracks  resembling  thoseshown  resultine  from  deep  etching  may  be  produced  "spontaneously"  in  serv- 
ice in  metals  highly  stressed  internally,  (a)  Section  of  a  i-inch  manganese  bronze  rod  etched  with  an 
acidulated  aqueous  solution  of  mercurous  nitrate  (65  g  mercurous  nitrate,  15  cm3  nitric  acid  per  liter).  X  i. 
(b)  Hardened  steel  balls  which  split  open  when  deeply  etched  with  concentrated  hydrochloric  acid.  X  2 

The  cracking  of  the  metals  illustrated  above  when  subjected  to 
the  proper  etching  reagents,  and  the  similar  deterioration,  "  season 
cracking,"  which  may  occur  spontaneously  in  service,  are  not  to 
be  regarded  simply  as  a  result  of  structural  variations.  However, 
the  distortion  of  the  structure  in  the  cold-worked  metals  in  which 
such  conditions  occur  is  very  pronounced  and  undoubtedly  is 

11  P.  D.  Merica  and  R.  W.  Woodward,  Failure  of  Brass:  i.— Microstructure  and  Initial  Stresses  in 
Wrought  Brasses  :>f  the  Type  60  Per  Cent  Copper  and  40  Per  Cent  Zinc,  B.  S.  Tech.  Papers,  No.  82. 


Structure  and  Properties  of  Metals  23 

contributory  in  a  very  large  degree  to  the  unusual  behavior  of  the 
metal. 

The  various  features  revealed  by  the  microscopic  examination 
of  metals  may  be  summarized  under  the  following  types : 

1 .  Chemical  unhomogeneity,  the  result  of  segregation,  liquation, 
decarburization,  cementation,   and  similar  causes.     In  wrought 
metals  lack  of  complete  chemical  homogeneity  often  serves  the 
useful  purpose  of  furnishing  a  record  of  the  plastic  flow  of  the 
metal    during   the    various    manufacturing   operations.     This   is 
generally  revealed  by  etching. 

2.  Crystalline  heterogeneity,  resulting  from  the  rate  of  cooling 
and  local  variations  in  the  cooling.     Local  overheating  may  also 
contribute  to  this.     Etching  with  ammonium  persulphate  is  ad- 
mirable for  revealing  crystalline  heterogeneity  in  both  steel  and 
copper  alloys. 

3.  Mechanical  nonuniformity,  or  presence  of  internal  stresses. 
When  the  condition  is  very  severe  it  may  be  revealed  by  deep 
etching  with  mercury  solutions  for  copper  alloys  and  concen- 
trated acids  for  steels. 

4.  Physical    unsoundness,    blowholes,    porosity,    "flakes,"    in- 
ternal discontinuities,  etc.     X-ray  and  the  magnetic  examina- 
tion, in  addition  to  visual  examination,  may  be  used  to  show  such 
features. 

3.  MICROSCOPIC  EXAMINATION 

(a)  SELECTION  OF  TYPICAL  SPECIMENS 

The  purpose  of  the  microscopic  examination  will  usually  be 
the  deciding  factor  in  the  selection  of  the  specimens.  However, 
there  are  certain  principles  which  may  be  mentioned  governing 
the  sampling  of  materials  for  examinations  of  this  kind.  Areas 
of  segregation  (as  determined  by  a  preliminary  macroscopic 
examination)  must  be  carefully  avoided  if  a  structure  representa- 
tive of  the  alloy  is  desired  for  observation;  on  the  other  hand, 
specimens  should  be  taken  from  the  zone  of  segregation  if  the 
subject  of  impurities  is  of  prime  consideration.  Usually,  for 
alloys  which  have  been  mechanically  worked,  specimens  should 
be  chosen  so  as  to  represent  the  changes  brought  about  by  the 
working;  that  is,  sections  parallel  to  and  others  perpendicular  to 
the  direction  of  working  should  be  examined.  In  materials  which 
proved  defective  in  service,  some  specimens  at  least  should  be 
taken  immediately  adjacent  to  the  fracture,  or  defects.  In  such 
cases  as  these,  in  which  the  metal  up  to  the  extreme  edge  of  the 


24  Circular  of  the  Bureau  of  Standards 

specimen  must  be  examined,  the  specimen  must  be  protected  in 
some  way  during  the  process  of  grinding  and  polishing.  A  method 
for  doing  this  is  described  below. 

(6)  PREPARATION  OF  SPECIMENS 

During  the  preparation — that  is,  the  grinding  and  polishing — 
of  the  specimens  for  microscopic  examination  the  edges  are 
rounded  and  beveled  off  somewhat  unless  care  is  taken  to  pro- 
tect them.  In  many  cases  such  a  precaution  is  absolutely  neces- 
sary, for  example,  easehardened  steels,  coated  metals,  the  frac- 
tured ends  of  test  bars,  etc.  Often  specimens  available  are  too 
small  for  use  unless  held  in  some  kind  of  a  matrix.  For  all  such 
cases  it  is  very  convenient  to  coat  the  specimen  with  a  heavy 
deposit  of  electrolytic  copper,  to  mount  it  in  a  matrix  of  some 
kind,  and  then  to  cut  and  polish  a  section  through  the  resulting 
duplex  specimen. 

The  common  acid-sulphate  bath,  consisting  of  250  g  crystal- 
lized copper  sulphate,  50  g  (approximately  30  cm3)  concentrated 
sulphuric  acid,  and  water  sufficient  to  make  one  liter  of  solution, 
may  be  used  for  the  solution  in  which  the  specimens  are  copper- 
plated.  For  iron,  steel,  zinc-coated  articles,  and  the  like,  it  is 
necessary  to  plate  the  specimen  with  a  thin  coating  first  in  a 
slightly  alkaline  bath  before  using  the  acid-sulphate  bath,  other- 
wise a  spongy  deposit  will  result  if  the  specimen  is  inserted  di- 
rectly into  the  acid-sulphate  bath.  This  preliminary  coating 
may  be  very  conveniently  deposited  by  means  of  a  cuprous 
cyanide  bath.  Such  a  solution  may  be  made  as  follows:  The 
precipitate  formed  by  mixing  aqueous  solutions  of  300  g  each  of 
copper  sulphate  and  sodium  carbonate  (crystallized)  is  added  to 
5  liters  of  water.  This  is  then  reduced  to  the  cuprous  condition 
by  adding  i  liter  of  water  containing  200  g  sodium  bisulphite. 
A  more  convenient  way,  however,  is  to  bubble  sulphur  dioxide 
gas  from  a  cylinder  of  the  liquefied  gas  through  the  liquid  and  the 
suspended  precipitate  until  the  color  indicates  that  the  reduction 
is  complete.  One  liter  of  water  in  which  250  g  crystallized 
sodium  carbonate  have  been  dissolved  is  added,  this  is  followed 
by  a  liter  of  water  containing  250  g  potassium  cyanide  in  solu- 
tion, and  the  whole,  which  upon  shaking  should  give  a  clear 
solution,  is  diluted  to  a  volume  of  10  liters.  Cuprous  cyanide  for 
the  direct  preparation  of  the  solution  may  be  purchased  from 
most  dealers  of  electro  platers'  supplies. 


Structure  and  Properties  of  Metals  25 

In  many  cases,  particularly  with  copper  alloys,  it  is  very 
desirable  to  deposit  a  preliminary  layer  of  nickel  before  the 
heavy  copper  layer  is  added,  so  that  there  will  be  no  uncertainty 
as  to  the  line  of  demarcation  between  specimen  and  coating. 
This  has  been  found  very  necessary  in  such  problems  as  the 
microscopic  study  of  corrosion  of  brasses  and  bronzes.  A  very 
convenient  solution  may  be  made  up  as  follows :  Nickel  ammonium 
sulphate,  90  g;  ammonium  chloride,  22.5  g;  boric  acid,  15  g;  and 
sufficient  water  to  produce  a  volume  of  i  liter. 

After  the  specimens  are  heavily  plated  they  are  mounted  as 
follows:  A  short  section  of  brass  or  other  tubing  is  filed  smooth 
on  one  end  and  placed,  with  this  smoothed  end  down,  upon  a 
block  of  graphite.  The  specimen  is  placed  inside  the  tube  with 
the  face  to  be  examined  down  and  then  backed  up  with  a  matrix. 
Molten  50-50  lead-tin  solder  is  used  a  great  deal  where  gentle 
heating  of  the  specimen  is  not  objectionable.  In  the  case  of 
hardened  steels,  etc.,  an  alloy  of  very  low  melting  point,  for 
example,  Rose's  alloy  (lead,  28  per  cent;  tin,  22  per  cent;  and 
bismuth,  50  per  cent;  melting  point,  approximately  95°  C)  or 
Wood's  alloy  (lead,  25  per  cent,  tin,  13  per  cent;  bismuth,  50 
per  cent,  and  cadmium,  12  per  cent;  melting  point  65°  C)  may 
be  used.  A  much  cheaper  substitute  that  is  very  suitable  for 
almost  all  classes  of  work  is  made  by  mixing  litharge  (PbO)  and 
glycerin  in  the  form  of  a  thick  paste.  This  is  poured  around  the 
specimen  and  will  set  and  form  a  very  hard  surface  which  does 
not  interfere  with  the  polishing  of  the  metal.  It  is  sometimes 
necessary,  however,  to  detach  the  specimen  from  such  a  matrix 
after  polishing  before  etching ;  this  is  particularly  true  if  alkaline 
reagents  such  as  sodium  picrate  are  used.  Often  it  is  very  con- 
venient to  cut  a  cavity  in  the  graphite  block  so  that  the  specimen 
which  is  inserted  into  the  depression  will  project  beyond  the  face 
of  the  solidified  matrix  which  holds  it.  The  projecting  portion 
is  then  cut  off  with  a  fine  hack  saw  and  the  metal  ground  and 
polished  without  removing  as  much  of  the  whole  as  would  have 
been  necessary  if  the  specimen  had  been  mounted  flat  within  the 
ring. 

In  Fig.  ii  some  applications  of  the  plating  and  mounting  of 
metallographic  specimens  are  shown.  / 

The  subject  of  the  mechanical  preparation  of  metal  surfaces  for 
microscopic  observation  has  been  discussed  in  detail  in  all  of  the 
reference  works  on  the  subject  of  metallography  and  need  not  be 
repeated  here.  The  precaution  that  should  be  always  borne  in 


26 


Circular  of  the  Bureau  of  Standards 


FIG.  ii. — Illustrations  of  the  proper  method  of  mounting  -very  small  metallic  specimens 
for  microscopic  examination 

(a)  Polished  face  of  a  specimen  prepared  for  micnscopic  examination.  The  material  to  be  examined, 
copper  wire  0.015  inch  diameter,  was  electrolytically  plated  with  nickel  and  with  copper,  embedded  in  a 
matrix  of  molten  tin-lead  alloy  (solder),  and  a  section  of  the  duplex  specimen,  after  cooling,  was  prepared. 
X  i.  (6)  Cross  section  of  one  of  the  small  wires  of  a.  The  white  ring  is  the  layer  of  nickel,  outside  of  this  a 
heavy  layer  of  electrolytic  copper  was  deposited.  X  100.  (c)  A  wire  similar  to  that  of  b  was  rolled  to  a 
strip  0.003  inch  in  thickness  before  mounting  in  the  manner  described.  X  100.  (d)  Small  fragment  of  a 
lathe-turning  of  mild  steel  electroplated  with  copper  and  mounted.  X  100.  Etching  reagents :  b  and  c, 
Ammoniacal  solution  of  copper  ammonium  chloride;  d,  5  per  cent  alcoholic  solution  of  picric  acid 


Structure  and  Properties  of  Metals  27 

mind  as  a  guide  in  this  work  is  that  a  buffed  surface  is  not  suitable. 
A  cutting  action  must  be  maintained  throughout.  The  number 
of  steps  varies  considerably  with  different  alloys  and  with  different 
workers.  The  following  method  has  been  found  very  suitable  at 
this  Bureau  for  the  preparation  of  ordinary  specimens — steels, 
brasses,  etc. 

Grinding  motors  with  a  variable  speed  of  500  to  i  ,200  rpm  which 
carry  aluminum  disks  at  each  end  of  the  armature  shaft  are  used. 
To  these  disks  emery  paper  is  attached;  the  number  of  grades  of 
paper  used  varies,  for  the  greater  part  of  the  work  three  steps 
having  been  found  sufficient  after  the  preliminary  smoothing  of 
the  specimen  with  a  file,  surface  grinder,  emery  wheel,  etc.  These 
are  domestic  (American)  emery  paper  >a,  Hubert  (French)  i  G, 
and  Hubert  o  or  oo.  It  is  very  essential  that  the  specimen  have 
a  flat  surface  at  the  start,  otherwise  the  process  of  polishing  is 
very  long,  tedious,  and  expensive  in  time  and  supplies. 

The  grinding  of  the  specimen  with  the  finest  emery  paper  is 
followed  by  wet  grinding  on  cloth-covered  disks,  kersey  being 
used  in  preference  to  the  usually  recommended  broadcloth.  As  a 
fine  abrasive  "3  F  alundum"  and  "S  F  emery  flour"  are  very 
suitable.  The  final  polishing  is  done  -on  a  similar  cloth-covered 
disk  by  means  of  levigated  alumina. 

The  lack  of  a  uniform  method  among  manufacturers  for  desig- 
nating the  different  grades  of  emery  papers,  emery  powders,  etc., 
renders  it  difficult  to  describe  concisely  the  preparation  of  the 
surface.  For  this  reason  there  is  shown  in  Fig.  12  the  surface 
condition  resulting  from  the  use  of  various  grades  of  papers  and 
abrasive  powders  upon  a  specimen  of  annealed  medium-carbon 
steel. 

The  procedure  given  above  must  often  be  changed  to  suit  the 
alloy;  for  example,  aluminum  is  best  prepared  by  cutting  on  a 
fine  file  under  kerosene,  grinding  at  low  speed  on  the  finer  grades 
of  emery  paper  which  are  kept  wet  with  alcohol,  kerosene,  or 
the  like,  and  then  finished  by  hand  on  a  cloth-covered  polishing 
block  with  fine  levigated  alumina.  Other  soft  alloys  and  metals 
can  be  prepared  similarly. 

For  the  examination  of  some  unusual  features  of  structure  it 
is  sometimes  desirable  to  carry  out  the  entire  process  by  hand. 
These  cases,  however,  are  quite  rare.  It  has  recently  been  shown 13 
that  the  working  of  the  surface  during  the  process  of  grinding 

11 H.  C.  H.  Carpenter  and  C.  F.  Elam,  Crystal  growth  and  Recrystallization  in  metals.  J.  Inst.  of  Metals. 
No.  a;  1920. 


28 


Circular  of  the  Bureau  of  Standards 


FIG.  12. — Appearance  of  the  face  of  a  specimen  of  mild  steel  in  different  stages  of  prepa- 
ration for  microscopic  examination.     X  100 

Abrasive  materials  used:  (a)  American  emery  paper,  manufacturer's  number,  'A;  (6)  American  emery 
paper,  manufacturer's  number,  oo;  (c)  French  (Hubert)  emery  paper,  manufacturer's  number,  iG;  (d) 
Emery  paper  similar  to  c,  manufacturer's  number,  o;  (e)  Emery  paper  similar  to  c,  manufacturer's  number, 
oo;  (/)  "  Alundum"  powder,  manufacturer's  number,  3F;  (?)  Emery  flour,  manufacturer's  number,  SF; 
(h)  "Levigated"  alumina.  In  stages  a  to  e,  inclusive,  the  specimen  was  polished  dry;  stages/,  g,  and  h 
were  carried  out  on  kersey-covered  discs  which  were  kept  moist 


Structure  and  Properties  of  Metals  29 

and  polishing  is  sufficient  to  cause  recrystallization  of  the  surface 
metal  in  the  case  of  some  of  the  softer  metals  and  alloys.  The 
real  structural  condition  of  the  metal  can  be  revealed  only  by  a 
series  of  alternate  polishings  and  etchings;  the  supplementary 
etching  during  the  process  of  preparation  is  for  the  purpose  of 
removing  the  recrystallized  metal  at  the  surface. 

(c)  METHODS  OF  ETCHING 

For  the  microscopic  examination  of  most  alloys  the  polished 
surface  of  the  specimen  must  be  properly  etched  in  order  to 
reveal  the  structure,  although  a  preliminary  examination  of  the 
polished  but  unetched  material  should  be  made  because  some 
features,  for  example,  inclosures,  are  best  seen  and  recognized 
when  the  specimen  is  unetched  or  at  least  only  slightly  etched. 
While  there  are  certain  general  principles  which  must  be  observed 
in  the  choice  of  a  suitable  etching  reagent  for  any  particular 
alloy,  there  is  without  doubt  more  chance  here  for  individual 
preference  than  in  any  other  phase  of  the  study  of  metal  structures. 
A  comprehensive  study  of  the  action  of  the  various  metallographic 
etching  reagents  upon  different  types  of  metals  and  alloys  is  in 
progress  at  this  Bureau. 

The  results  already  obtained  "  indicate  that  oxidation  is  of 
fundamental  importance  in  the  successful  etching  of  copper  and 
the  copper-rich  alloy  and  presumably  also  for  a  great  many  of 
the  other  alloys.  Many  reagents  which  ordinarily  have  only  a 
very  slight  solvent  action  upon  copper  or  copper  alloys  may  be 
used  successfully  for  purposes  of  etching  if  the  action  is  intensified 
by  oxidation.  This  may  be  done  either  by  additions  of  oxidizing 
reagents  or  by  passing  oxygen  gas  through  the  etching  reagent 
while  the  specimen  is  immersed. 

In  some  cases  the  use  of  two  or  more  different  types  of  etching 
in  succession  for  the  same  specimen  is  desirable,  no  attempt  being 
made  to  remove  the  results  of  the  first  etching  before  the  second 
reagent  is  used.  The  second  or  supplementary  etching  may  be 
either  for  the  purpose  of  making  prominent  a  constituent  or 
structural  condition  not  plainly  revealed  by  the  first  reagent 
(Fig.  1 8  a  and  6),  or  for  removing  a  surface  film  caused  by  the 
products  of  first-etching  reaction.  This  is  often  necessary  in 
etching  with  silver  nitrate,  as  an  obscuring  film  of  silver  precip- 
itated over  the  face  of  the  specimen  must  be  removed  before  the 

"  H.  S.  Rawdon  and  M.  G.  Lorentz,  Metallographic  Etching  Reagents:  I,  for  Copper,  B.  S.  Sci.  Papers 
No.  399- 


30  Circular  of  the  Bureau  of  Standards 

true  structure  of  the  etched  metal  is  revealed.  However,  this 
may  often  be  removed  with  a  moist  cotton  swab.  Likewise,  in 
the  etching  of  aluminum  an  obscuring  black  deposit  resulting 
from  the  action  of  the  etching  reagent  must  be  removed  by 
immersing  the  etched  specimen  in  a  suitable  second  solution. 

Below  is  given  a  list  of  most  of  the  common  reagents  in  use  at 
the  Bureau  of  Standards.  To  give  anything  like  a  complete  list 
or  instructions  for  use  is  manifestly  impossible. 

TABLE  1.— Metallographic  Etching  Reagents  for  Revealing  Microstructure 


Method  of  etching 


Copper  and  copper-rich  al- 
loys (brass,  bronze,  alumi- 
num bronze) 


Aluminum  and  aluminum- 
rich  alloys 


An  ammoniacal  or  an  acid  oxidiz- 
ing solution 


Ammoniacal  solution  of  copper- 
ammonium  chloride 

Oxidizing  acids 

Aqueous  solution  of  silver  nitrate. 

Concentrated     ammonium     hy- 
droxide 
Heat  tinting 

Hydrofluoric  acid 


Aqueous   solution   of  sodium   or 
potassium  hydroxide 


Lead ..    Nitric  acid 


Lead  and  tin  alloys,  including 
"  white  metals  " 


ilute  nitric  acid 


Dilute  hydrochloric  acid 
[Aqueous  solution  of  silver  nitrate 


I  Concentrated  nitric  acid . . 


Nickel-rich  alloys  (Monel 
metal,  Benedick  nickel, 
nickel  brass,  invar,  etc.) 


Electrolytic  etching 
{ Ferric  chloride 

Dilute  sulphuric  acid  containing 
an  oxidizer  (hydrogen  peroxide 
or  potassium  permanganate) 
I  Concentrated  hydrochloric  acid 


(Same  as  for  nickel 

:{ Ferric  chloride 

j  [Ammonium  persulphate 


I  (Sodium    hydroxide;    mixture    of 
chromic  and  nitric  acid;  am- 

Zinc  and  zinc-rich  alloys.....  ,<    monium  chloride;  iodine;  am- 
monium persulphate 
(Electrolytic  etching 

Gold,  platinum,  and  "noble"  j  Aqua  regia 
metals  and  alloys 


Silver    and    its    alloys    with 
copper 


Wrought  iion,  "pure"  iron. 


Nitric  acid;  ammonium  persul- 
phate  solution 


1  Picric  acid.... 
ICupric  reagent . 


Suitable  oxidizers  for  use:  Hydrogen 
peroxide,  ammonium  persulphate, 
potassium  permanganate,  potassium 
dichromate,  chromic  acid,  ferric 
chloride 

Electrolytic  in  its  nature 

Nitric  acid  and  chromic  acid 

The  film  of  precipitated  silver  must  be 

removed 
Accompanying  oxidation  is  necessary  to 

produce  satisfactory  results 
Valuable  for  certain  bronzes 

An  approximately  10  per  cent  aqueous 
solution  is  used,  a  supplementary  im- 
mersion in  concentrated  nitric  acid  or 
in  chromic  acid  may  be  necessary  to 
clean  the  surface 

0.1  per  cent  aqueous  solution  is  suitable 
for  revealing  the  constituents,  more 
concentrated  sulutions  for  grain 
boundaries 


Alone  or  with  an  addition  of   chromic 
acid 


Used  alone  or  in  a  solution  of  glacial 
acetic  acid,  approximately  nitric  acid 
50,  acetic  acid  40,  water  10  per  cent 


A  long  etching  period  is  required,  one 
hour  or  so 


Same  as  for  nickel 


Alcoholic  solutions,  approximately  1  per 
cent 


2  per  cent  alcoholic  solution,  commonly 

used 

5  per  cent  alcoholic  solution 
To   reveal   phosphorus   banding,    and 

similar  structural  features 


Structure  and  Properties  of  Metals  31 

TABLE  1. — Metallographic  Etching  Reagents  for  Revealing  Microstructure — Contd. 


Material 

Method  of  etching 

Remarks 

Carbon  steels 

[Nitric  acid,  picric  acid,  and  cupric 
reagent,  as  above 
•{Hot  alkaline  sodium  picrate  or 

Used  to  color  cementite 

Alloy  steels  

other  ozidizers 
[Hydrochloric  acid  

[Same    reagents    as    for    carbon 
steels  above 
\2  per  cent  alcoholic  picric  acid, 
very  prolonged  etching 
1  Sodium  picrate  and  other  oxidizers 

(Picric  acid,  or  nitric  acid  as  above 
Heat  tinting  

1  per  cent  alcoholic  solution 

For  revealing  grain  boundaries 
For  steels  showing  free  carbide 

To  identify  iron  phosphide  and  man- 

[Sodium picrate 

ganese  sulphide 

FIG.  13. — Constitutional  diagram  of  the  copper-zinc  series  of  alloys 

The  microstructural  features  of  the  different  types  of  alloys,  the  composition  of  which  is  indicated  by 
the  circles  at  the  base  of  the  diagram,  are  shown  in  Pig.  14 


Circular  of  the  Bureau  of  Standards 


FIG.  14. — Microstructure  of  the  principal  types  of  brasses,  that  is,  alloys  of  the  copper- 
zinc  series.     X  100 

Note  the  alternation  of  alloys  of  simple  and  of  duplex  structure  as  the  composition  is  varied.  The  alloys 
d  to  h,  inclusive,  are  of  very  little  use  industrially.  The  approximate  composition  is  indicated  on  the  dia- 
gram. Fig.  13.  (a)  a  brass,  annealed  after  cold  work,  etched  with  acidified  aqueous  solution  of  ferric  chlo- 
ride; (b)  a-p  brass,  hot-rolled,  yellow  matrix  of  beta  containing  reddish  figures  of  alpha;  etched  with  a 
dilute  solution  of  sulphuric  acid  containing  potassium  dichromate;  (c)  ft  brass,  cast,  golden  yellow  in  color, 
etched  with  an  ammoniacal  solution  of  copper  ammonium  chloride;  (d)  0-y  brass,  cast,  yellow  matrix  of 
beta  containing  silvery  gray  crystallites  of  gamma;  etched  with  aqueous  solution  of  ammonium  persul- 
phate; (e)  7  brass,  cast;silvery  gray  in  color,  hard  and  brittle;  etched  asin</;(/)  7- « brass,  cast;  matrix  of  y 
containing  the  eutectoid  of  y  and  <;  etched  as  in  d;  (g)  f  brass,  cast,  slightly  purple  when  etched;  etching 
reagent,  ammonium  hydroxide  and  ammonium  persulphate;  (A)  f-i)  brass,  zinc-rich  matrix  of  «  containing 
bright  unetched  crystallites  of  «;  etched  as  in  d 


Structure  and  Properties  of  Metals  33 

IV.  CONDITIONS  AFFECTING  STRUCTURE 
1.  CHEMICAL  COMPOSITION 

Of  all  the  factors  which  determine  the  structure  of  an  alloy, 
chemical  composition  is  without  doubt  the  most  important. 
Not  only  do  alloys  of  entirely  different  composition  differ  in 
structure,  but  alloys  of  varying  proportions  of  the  same  metals 
show  structural  variations  according  to  the  percentage  composi- 
tion which  are  often  as  marked  as  if  they  were  composed  of  en- 
tirely different  component  metals.  To  discuss  adequately  these 
structural  variations  due  to  composition  would  necessitate  a 
lengthy  review  of  the  subject  of  phase  rule  and  the  different 
classes  of  alloys  on  the  basis  of  the  constitutional  diagram  of  the 
various  systems.  Such  a  review  is  neither  necessary  nor  desir- 
able here.  A  brief  reference  to  the  copper-zinc  alloys  will  suffice 
to  illustrate  the  point.  In  Fig.  13  is  given  the  constitutional  or 
structural  diagram  of  this  series,  in  which  is  shown  the  condi- 
tion which  obtains  for  every  composition  for  temperatures  from 
o°C.  up  to  that  of  the  molten  metal.  In  Fig.  14  micrographs  of 
typical  copper-zinc  alloys  are  shown  to  illustrate  the  pronounced 
changes  which  occur  in  the  structure  at  room  temperatures  as 
the  composition  is  varied. 

A  similar  set  of  micrographs  is  given  in  Fig.  15  to  show  the 
change  occurring  in  annealed  steel  as  the  carbon  content  is  pro- 
gressively increased.  It  is  evident  from  Fig.  15  that  it  should 
be  possible  to  determine  the  carbon  content  of  such  material 
rather  accurately  from  careful  estimation  of  the  amount  of  the 
carbon-bearing  constituent  present  in  the  steel.  Such  a  method 
is  often  used  to  supplement  chemical  analysis  and  to  show  the 
magnitude  of  variations  in  composition,  which  the  ordinary 
method  of  sampling  does  not  permit  chemical  analysis  to  detect. 

As  already  mentioned  in  the  discussion  of  the  macrostructure 
of  metals,  segregation  in  steels  is  one  of  the  most  serious  defects 
to  which  this  class  of  metals  is  subject.  The  composition  of  the 
segregated  portion  of  a  steel  article  often  differs  widely  from  that 
of  the  unsegregated  portions.  This  is  shown  in  Fig.  16,  which 
represents  the  structure  of  a  streak  of  segregated  metal  from  the 
center  of  a  railhead  and  also  the  intermediate  and  the  outer  por- 
tions of  the  same.  The  need  of  an  examination  of  the  structure 
to  supplement  a  chemical  analysis  in  such  cases  is  very  evident. 

110580°— 22 3 


34 


Circular  of  the  Bureau  of  Standards 


.- 

•-  -l/r't 

;;•  «  .^t^ 

i-:f;J    •>r4*.sV--'i 


:  Kv 


FIG.  15. — ^licrostructure  of  typical  annealed  iron-carbon  alloys  (steels)  illustrating  the 
effect  of -variations  in  carbon  content  upon  structure 

(a)  Ferrite,  electrolytic  iron  melted  in  vacuo.  (6)  to  (g)  Steels  of  ferrite-pearlite  structure  containing 
progressively  increasing  amounts  of  carbon  as  follows:  6,  0.03  per  cent;  c,  0.07  per  cent;  d,  0.24  per  cent;  e, 
0.32  per  cent;/,  0.59  per  cent;  g,  0.68  per  cent.  (/;)  Eutectoid  steel,  0.85  per  cent  carbon,  (i)  to  (0  Steels  of 
pearlite-cementite  structure  containing  progressively  increasing  amounts  of  carbon  as  follows:  i,  1.14  per 
cent;/,  1.14  per  cent;  k,  1.45  per  cent;  /,  1.70  per  cent.  X  100,  except  g,  h,  and  j,  X  500.  Etching  reagents, 
except  for  i,  2  per  cent  alcoholic  solution  of  nitric  acid,  for  »'  hot  alkaline  solution  of  sodium  picrate 


Structure  and  Properties  of  Metals 


35 


'fl 


FIG.  16. — Variations  in  the  microstructure  of  rail  steel  caused  by  segregation 

The  structure  shown  in  d  is  indicative  of  very  brittle  material  and  the  presence  cf  this  condition  in  the 
center  of  the  rail  head  was  responsible  for  the  failure  of  the  rail  in  service,  (a)  Cross  section  of  head  of  rail 
which  failed  in  service.  X  i.  (/>)  Structure  of  the  steel  comprising  the  greater  part  of  the  head,  pearlite 
and  ferrite.  X  too.  (c)  Structure  of  steel  from  near  the  center  of  the  head,  pearlite  with  traces  of  f  errites. 
X  100.  _  (d)  Structure  of  the  metal  adjacent  to  the  split  which  formed  in  the  head,  pearlite  with  films  of 
cementite  enveloping  the  grains.  The  metal  was  shattered  by  the  formation  of  numerous  intercrystalline 
cracks  under  the  load  to  which  it  was  subjected.  X  100.  Etching  reagent:  6  and  c,  2  per  cent  alcoholic 
solution  of  nitric  acid ;  d,  hot  alkaline  solution  of  sodium  picrate 


Circular  of  the  Bureau  of  Standards 


It  may  be  noted  in  passing  that  this  rail  failed  in  service  in  the 
track,  the  fundamental  cause  being  its  abnormal  segregation. 

It  often  happens  that  metals  and  alloys  show  certain  features 
of  structure  which  are  best  described  as  being  due  to  small  in- 
closures  of  foreign  material.  Such  inclosures,  however,  are  often 
foreign  only  in  the  sense  that  they  are  not  metallic.  They  are  a 
necessary  result  of  the  metallurgical  process  used  for  the  prepara- 
tion of  the  material.  The  ever-present  slag  of  wrought  iron 
(Fig.  52)  is  an  example.  Such  slag  threads  are  characteristic  of 
this  material,  and  their  presence  or  absence  is  often  used  as  a 
criterion  in  disputed  cases  in  deciding  upon  the  nature  of  the 
material.  The  foreign  inclosures  may  also  result  from  additions 
made  to  the  metal  in  course  of  preparation  for  improving  its 

properties  by  some  chem- 
ical reaction,  for  example, 
deoxidation  and  similar 
reactions.  The  products 
of  the  reaction  are  often 
retained  in  part  by  the 
metal  after  solidification 
and  form  a  characteristic 
feature  of  the  structure. 
Fig.  17  shows  an  inclu- 
sion in  steel  which  re- 
sulted from  an  addition 
of  titanium  to  the  metal. 
The  pink  color  and  shape 
are  quite  characteristic 
of  inclosures  of  this  kind. 
Some  of  the  foreign  inclusions  are  under  certain  conditions 
decidedly  injurious  to  the  metal  in  which  they  occur;  that  is, 
they  cause  its  mechanical  properties  to  be  very  inferior  to  what 
they  would  be  otherwise.  Sulphur  in  steel  is  a  well  known 
example  of  this.  In  the  form  of  ferrous  sulphide,  because  of  the 
form  in  which  it  is  distributed  (as  thin  films  enveloping  the 
grains),  it  renders  the  steel  almost  unworkable  at  a  red  heat. 
This  can  be  readily  overcome,  however,  by  the  proper  addition 
of  manganese  to  the  molten  steel  at  the  proper  time.  Fig.  18 
shows  the  characteristic  appearance  of  the  sulphur  in  steel  when 
in  the  form  of  ferrous  sulphide  and  also  when  it  has  been  con- 
verted into  manganese  sulphide.  The  inclusions  of  manganese 


FIG.  17. — Characteristic  appearance  of  an  inclu- 
sion caused  by  the  addition  of  titanium  to  steel. 
X  1000 
Etching  reagent,  2  per  cent  alcoholic  solution  of  nitric  acid 


Structure  and  Properties  of  Metals  37 

sulphide  are  not  particularly  harmful  in  the  metal;  that  is,  no 
more  so  than  any  similar  inclusions  are. 


FIG.  18. — Alicrostructure  of  low  carbon  steel,  illustrating  the  forms  in  "which  sulphur  may 

occur 

Xote  the  dark -colored  films  enveloping  the  grains  a  and  6.  Sulphide  in  this  form  is  very  detrimental  to 
the  properties  of  steel,  (a)  Longitudinal  section  of  the  segregated  center  of  a  2-inch  round  intended  for 
chains;  it  was  impossible  to  forge  and  weld  the  material  satisfactorily  on  account  of  the  films  of  ferrous 
sulphide  enveloping  the  grains.  X  100.  (6)  Same  as  a.  X  500.  Etching  reagent,  2  per  cent  alcoholic 
solution  of  nitric  acid  followed  by  hot  alkaline  sodium  picrate.  (c)  Longitudinal  section  of  steel  rod  in- 
tended for  use  in  automatic  lathe.  The  sulphur  is  in  the  form  of  isolated  globules  of  manganese  sulphide. 
X  xoo.  (d)  Same  as  c.  Specimen  is  unetched.  X  500 

A  striking  illustration  of  the  effect  of  relatively  slight  changes 
in  the  chemical  composition  upon  the  structure  of  an  alloy  is  af- 


38  Circular  of  the  Bureau  of  Standards 

forded  by  cast  iron.  When  slowly  cooled  from  the  molten  state, 
the  metal  assumes  the  form  commonly  known  as  gray  iron,  a  large 
proportion  of  its  carbon  being  in  the  form  of  graphite.  By  chilling 
the  metal  when  cast,  the  form  known  as  white  iron  results,  the 
metal  being  in  a  state  of  unstable  equilibrium.  In  this  case  most 
of  the  carbon  is  retained  in  the  combined  form ;  that  is,  as  cement- 
ite — the  hard,  brittle  constituent.  The  relative  silicon  content 
of  different  cast  irons  has  a  pronounced  effect  upon  the  structure 
resulting,  even  where  all  are  cooled  at  the  same  rate.  It  is  a  well- 
established  fact  that  an  increase  of  silicon  favors  graphitization ; 
that  is,  induces  stable  equilibrium  even  with  rather  rapid  cooling, 
while  a  low  silicon  content  permits  the  alloy  to  retain  the  unstable 
condition.  For  this  reason  the  silicon  content  of  cast  iron  is  pur- 
posely varied  by  the  foundry  man ;  for  ordinary  castings  for  which 
gray  iron  is  desired,  the  silicon  content  is  raised  to  a  relatively 
high  value,  for  example,  2  per  cent  and  more.  On  the  other  hand, 
when  a  white  iron  is  desired  either  for  chilled  castings  to  be  used  as 
such  or  for  the  production  of  malleable  castings,  the  silicon  content 
is  kept  relatively  low,  for  example,  approximately  0.50  per  cent. 

2.  TEMPERATURE 

The  temperature  to  which  a  metal  or  alloy  is  heated,  that  is, 
after  it  has  been  formed,  has  in  most  cases  a  marked  influence 
upon  its  structure.  These  structural  changes  dependent  upon 
heating  may  be  conveniently  discussed  under  the  following  head- 
ings: Equilibrium  changes,  grain  growth,  and  phase  changes. 

(a)  EQUILIBRIUM  CHANGES 

Practically  all  metals  and  alloys  when  cast  are  far  from  being  in 
a  condition  of  structural  equilibrium  such  as  the  phase  rule 
predicates  for  the  given  conditions  of  composition  and  temperature. 
Although  such  conditions  are  often  accentuated  by  rapid  cooling 
during  the  process  of  casting  of  the  alloy,  slow  cooling  in  itself 
does  not  necessarily  result  in  structural  equilibrium.  Metalsj 
unless  exceptionally  pure,  and  alloys  normally  show  a  cored  or 
dendritic  structure  when  in  the  cast  state.  This  is  a  natural  con- 
sequence of  the  selective  process  of  freezing  by  which  they  solidify. 
In  each  crystal  a  branched  or  treelike  core  which  is  relatively  rich 
in  the  element  of  highest  melting  point  is  first  formed,  and  non- 
soluble  impurities  collect,  and  constituents  of  lower  melting 
point  form  in  the  interstices  between  the  branches,  the  average 
composition  of  the  material  at  such  points  being  quite  different 


Structure  and  Properties  of  Metals 


39 


from  that  of  the  "core."  Thus,  every  grain  or  crystal  of  the  cast 
metal  or  alloy  is  nonhomogeneous  in  its  composition,  although  the 
average  composition  of  the  whole  grain  is  approximately  that  of 
the  average  for  the  alloy.  Fig.  19  illustrates  this  condition. 
When  an  alloy  showing  such  a  "cored"  structure  is  heated  for  a  time 


.1 


FIG.  19.— Characteristic  appearance  of  cast  alloys,  showing  a  dendritic  structure 

(a)  Nickel  brass,  a  one-constituent  alloy.  X  100.  The  castings  were  defective  on  account  of  the  inter- 
crystalline  cracks  which  formed  during  casting.  Etching  reagent,  concentrated  ammonium  hydroxide. 
(6)  Zinc  bronze,  a  two-constituent  alloy.  Approximate  composition:  Copper,  88  per  cent;  tin,  10  per  cent; 
zinc,  2  per  cent;  very  slowly  cooled  from  the  molten  state  X  5.  (c)  Same  as  b.  X  100.  Etching  reagent 
6  and  c,  ammonium  hydroxide  and  hydrogen  peroxide 

at  a  relatively  high  temperature,  the  principal  effect  is  to  erase  the 
structural  pattern  by  allowing  diffusion  to  take  place  within  the 
body  of  each  crystal  so  that  chemical  homogeneity  is  approached. 
The  changes  which  take  place  in  a  cast  alloy  upon  heating  are 
shown  in  Fig.  20,  which  represents  a  bronze  as  cast  and  similar 


40  Circular  of  the  Bureau  of  Standards 

ones  after  prolonged  heating.     Unless  such  material  has  been  sub- 
jected   to    other    conditions,    that    is,   straining    by    mechanical 


FIG.  20. — Microstruclural  changes  in  a  cast  alloy  caused  by  prolonged  heating.     X  IOO 

Note  the  gradual  disappearance  of  the  dendritic  pattern  as  a  result  of  annealing  the  cast  alloy,  (a)  Cast 
zinc-bronze.  Approximate  composition :  Copper,  88  per  cent;  tin,  10  per  cent;  zinc,  2  per  cent.  As  cast, 
the  alloy  contains  two  constituents  and  has  a  pronounced  dendritic  structure.  (6)  Alloy  similar  to  a, 
heated  8  hours  at  400°  C.  Diffusion  has  taken  place  to  some  extent,  (c)  Alloy  similar  to  a,  heated  8  hours 
at  600°  C.  The  eutectoid  has  been  absorbed  by  the  matrix,  but  the  dendritic  pattern  is  still  obvious. 
(d)  Another  portion  of  specimen  c;  this  portion  of  the  alloy  consists  of  an  almost  homogeneous  solid  solu- 
tion. Etching  reagent,  ammonium  hydroxide  and  hydrogen  peroxide 

deformation  or  its  equivalent — excessively  rapid  cooling  from  a 
very  high  temperature,  change  of  grain  size  in  a  cast  alloy  does  not 


Structure  and  Properties  of  Metals  41 

take  place  upon  heating.  The  principal  effect  of  the  heat  upon 
the  chemically  nonhomogeneous  cast  material  is  to  render  it 
more  nearly  uniform  in  the  composition  by  permitting  diffusion 
and,  in  some  cases,  solution  of  certain  constituents  to  occur,  thus 
allowing  chemical  equilibrium  to  be  brought  about. 

Other  effects  of  heat  upon  the  structure  of  alloys  which  may  be 
briefly  mentioned  are  the  decomposition  of  a  compound,  as  is 
illustrated  by  the  formation  of  graphite  from  cementite  during 
the  heating  of  white  cast  iron  and  in  some  cases  in  high-carbon 
steels  (Fig.  21),  the  mechanical  break-up  of  eutectics  and  eutec- 


. 

'   V 


FIG.  21.  —  Microstructure  of  high-carbon  steel  in  which  graphitization  has  occurred 

Each  of  the  irregular  black  spots  indicates  the  presence  of  graphite.  Such  steel  is  useless  for  the  purpose 
for  which  high-carbon  steel  is  generally  employed,  (a)  Tool  steel,  i  per  cent  carbon,  showing  specks  of 
graphite  which  have  formed  at  the  expense  of  the  combined  carbon;  each  speck  of  graphite  is  sur- 
rounded by  an  area  of  fcrritc;  percentage  of  graphite,  0.31.  X  100.  (6)  Same  specimen.  X  500.  Etch- 
ing reagent,  2  per  cent  alcoholic  solution  of  nitric  acid 

toids  by  the  coalescence  of  the  different  constituents  under  the 
action  of  heat,  illustrated  by  the  spheroidizing  of  pearlite  by  long- 
continued  heating  of  steel  just  below  the  A1  transformation 
temperature  (Fig.  34)  .  Other  cases  might  be  cited  ;  the  above 
are  sufficient,  however,  to  illustrate  the  typical  changes  which 
may  occur. 

(b)  GRAIN  GROWTH 

The  most  striking  change  occurring  in  the  structure  of  metals 
and  alloys  upon  heating  is  the  increase  in  grain  size  which  often 
occurs.  Such  a  change  in  grain  size  usually  necessitates  a  pre- 
liminary straining  of  the  material.  Cast  alloys,  at  least  those 


Circular  of  the  Bureau  of  Standards 


FIG.  22. — Microstructure  0/0.47  per  cent  carbon  steel  illustrating  the  structural  effect  of 
overheating  and  of  grain  refinement.      X  -TOO 

Note  how  the  very  large  grains  (6)  may  be  replaced  by  much  smaller  ones  (c)  by  proper  annealing  practice. 
The  mechanical  properties  are  correspondingly  improved  at  the  same  time,  (a)  Material  as  received  from 
the  manufacturer.  (6)  Material  similar  to  a,  heated  6  hours  at  uio°C  (2030°  F)  and  cooled  in  air.  (c)  Mate- 
rial b,  reheated  at  780°  C  (1440°  F)  for  3  hours  and  45  minutes  and  furnace-cooled.  Etching  reagent,  a  per 
cent  alcoholic  solution  of  nitric  acid 


Structure  and,  Properties  of  Metals  43 

which  involve  no  phase  change  upon  heating,  will  show  no  increase 
in  grain  size  even  after  several  months  heating,15  unless  the 
material  has  been  strained  in  some  way.  Since  so  many  metals 
and  alloys  are  subjected  to  mechanical  work  of  some  kind  in  their 
fabrication  or  to  other  conditions  in  special  cases  which  bring 
about  grain  growth  upon  subsequent  heating,  the  fact  is  often  lost 
sight  of  that  grain  growth  is  not  the  simple  result  of  heating  only, 
and  that  other  conditions  are  necessaiy  to  bring  it  about.  The 
latest  opinions  on  this  subject  are  given  in  the  reference  cited 
above. 

Fig.  22  b  shows  the  increase  in  grain  size  which  resulted  in  a 
steel  bar  by  improper  annealing.  The  distortion  of  the  material 
which  occurred  in  the  rolling  of  the  metal  as  well  as  the  fact  that 
the  material  is  subject  to  a  phase  change  upon  heating  accounts 
for  the  pronounced  grain  growth  which  occurred  upon  heating. 
A  coarsely  grained  condition  in  metals  is  usually  regarded  as  very 
undesirable  and  particularly  so  in  metals  which  may  be  subjected 
to  shock  or  similar  conditions  in  service.  An  investigation 16 
under  way  at  this  Bureau  has  for  its  object  the  determination  of 
the  mechanical  properties  of  steels  which  are  most  affected  by 
variations  in  grain  size  of  the  material. 

(c)  PHASE  CHANGES 

In  a  great  many  alloys  pronounced  structural  changes  occur 
upon  heating  to  certain  definite  temperatures  which  of  course 
vary  with  the  different  alloys  under  consideration.  Such  changes 
are  to  be  ascribed  to  phase  changes  or  transformations  within  the 
material,  such  as  are  indicated  in  many  of  the  constitutional 
diagrams  for  the  various  classes  of  alloys.  By  quickly  cooling 
an  alloy  which  exhibits  such  changes  from  a  temperature  some- 
what higher  than  that  at  which  the  transformation  occurs,  the 
structure  normally  existing  only  at  the  higher  temperature 
persists  to  a  large  extent  in  the  material  at  room  temperature, 
the  alloy  being  in  a  state  of  unstable  equilibrium. 

This  fact  is  of  very  great  industrial  importance ;  the  art  of  heat 
treatment  of  alloys,  particularly  steel,  for  high  mechanical  prop- 
erties depends  upon  this  fact.  Fig.  23  shows  the  structure  of  a 
low-carbon  steel  as  usually  observed  and  the  structure  of  the  same 
as  it  exists  at  a  temperature  somewhat  above  the  first  or  A1 

16  H.  C.  H.  Carpenter  and  C.  F.  Elam,  "  Grain  growth  and  recrystallization  of  metals,"  J.  Inst.  of  Metals, 
24,  1920. 

16  H.  S.  Rawdon  and  Emilio  Jimeno-Gil,  The  Relation  of  Brinell  Hardness  to  the  Grain  Size  of  Annealed 
Carbon  Steels,  B.  S.  Sci.  Papers,  No.  397. 


44 


Circular  of  the  Bureau  of  Standards 


FIG.  23. — Microstructure  of  0.18  per  cent  carbon  steel  as  it  exists  at  ordinary  temperatures 
and  at  relatively  high  temperatures 

(a)  Pearlite-ferrite  structure  of  the  material  at  ordinary  temperatures,  revealed  by  etching  with  2  per 
cent  alcoholic  solution  of  nitric  acid.  X  100.  (b)  Same  specimen  as  a;  high-temperature  structure  revealed 
by  heatiug  the  polished  specimen  in  vacuo  30  minutes  at  750°  C  (above  the  Ai  transformation) ,  and  cooling 
in  vacua  A  pronounced  volume  change,  which  is  opposite  in  its  character  to  that  caused  by  heat  alone, 
occurs  in  the  steel  during  its  transformation  at  the  critical  temperature.  The  deformation  of  the  polished 
surface  due  to  this  change  in  volume  reveals  the  extent  to  which  the  austenitic  solid  solution  resulting  from 
the  transformation  of  the  pearlite  merged  with  the  f errite  matrix.  X  100.  (c)  Same  as  b,  X  500 

transformation,  as  recorded  by  a  special  method  of  heat  etching 
by  heating  the  polished  specimen  in  vacuo.  By  quickly  cooling 
the  steel,  for  example,  by  quenching,  from  the  temperature  at 


Structure  and  Properties  of  Metals  45 

which  the  "high-temperature  structure"  was  examined,  the 
structural  condition  obtaining  at  that  temperature  would  be 
maintained  in  the  quickly  cooled  alloy;  pronounced  changes  in 
its  physical  properties  would  result  from  the  new  structural 
conditions.  Fig.  71  illustrates  the  same  phenomenon  in  a  non- 
ferrous  alloy. 

The  phase  changes  or  transformations  in  alloys  are  accompanied 
by  energy  (heat)  manifestations.  Hence  it  is  much  easier  to 
investigate  and  establish  the  temperatures  at  which  the  changes 


if 

nil 

$8*5 


FIG.  24. — Microstructure   of  cold-drawn   copper,   showing   the  mechanical  distortion   as 
sections  parallel  to  and  perpendicular  to  the  direction  of  working.     X  250 


Note  the  difference  in  the  grains  of  a  acd  6;  the  mechanical  properties  differ  in  the  longitudinal  and  trans- 
verse directions  in  a  manner  corresponding  to  the  difference  in  structure,  (a)  Longitudinal  section  of 
'/i-inch  trolley  wire;  (b)  Cross  section  of  same.  Etching  reagent,  concentrated  ammonium  hydroxideand 
hydrogen  peroxide 

occur  by  the  relatively  simple  means  of  "thermal  analysis"  rather 
than  by  the  tedious  and  complicated  methods  necessary  for  the 
determination  of  the  high- temperature  structure  of  the  material. 
Thermal  analysis  may  be  regarded  then  as  the  means  for  demon- 
strating the  structural  changes  which  occur  in  alloys  upon  heating 
as  well  as  the  energy  changes  which  accompany  them.  It  may  be 
noted,  however,  that  some  energy  changes  have  been  observed 
for  which  any  possible  accompanying  structural  change  is  so 
minute  as  to  be  beyond  the  range  of  the  methods  now  used  for 
observing  the  structure  of  metals. 


46  Circular  of  the  Bureau  of  Standards 

3.  WORKING  OF  METALS 

(a)  DISTORTION  OF  CRYSTALLINE  STRUCTURE 

One  of  the  most  potent  factors  affecting  the  structure  of  metals 
and  alloys  is  the  amount  of  mechanical  working  received  during 


FIG.  25. — Microstructure  of  brass  illustrating  progressive  stages  in  the  crystalline  distor- 
tion produced  by  cold-working     X  IOO 

Note  the  scries  of  parallel  lines  within  the  different  grains.  The  hardening  of  brass  hy  cold-working  it  is 
the  result  of  the  changes  occurring  within  the  individual  crystals,  as  indicated  by  these  lines,  (a) ,  (ti) ,  and 
(c)  show  three  stages  in  the  crystalline  distortion  of  cartridge  brass  (approximate  composition,  copper,  ,70 
per  cent;  zinc,  30  per  cent)  by  cold  working  the  alloy.  The  series  of  parallel  lines  within  any  one  crystal 
represent  the  planes  along  which  the  metal  "slips."  Etching  reagent,  ammoniacal  solution  of  copper 
ammonium  chloride 


Structure  and  Properties  of  Metals  47 

fabrication  after  casting.  Aside  from  the  part  played  by  the 
straining  of  metals  in  causing  recrystallization  and  grain  growth 
upon  subsequent  heating  as  explained  above,  the  structure  of  the 
metal  is  often  profoundly  changed  by  the  deformation  of  the 
material.  Thus  sections  of  a  metal  parallel  to  the  direction  of 
working  will  differ  very  materially  in  their  appearance  from  those 
perpendicular  to  the  same.  Such  sections  are  conveniently 
designated  as  longitudinal  and  transverse  sections,  respectively. 
In  Fig.  24  two  such  sections  of  cold-drawn  copper  are  shown  to 
illustrate  the  structural 
change  which  accompa- 
nies mechanical  working. 

As  is  to  be  expected, 
if  the  deformation  or 
working  has  been  carried 
out  upon  the  cold  mate- 
rial, the  distortion  of  crys- 
talline structure  will  be 
much  more  severe  than  if 
carried  out  upon  the 
metal  while  hot.  Thus, 
thin  cold-rolled  sheets 
and  fine  cold-drawn  wires, 
particularly  of  the  softer 
metals  and  alloys,  appear 
practically  "structure- 
less" when  examined  by 
the  usual  metallographic 
methods.  Excessive  cold 
working  of  the  surface  of  harder  metals,  for  example,  steels,  will 
sometimes  result  in  the  formation  of  a  hard  ' '  structureless ' '  sur- 
face layer.  The  accompanying  temperature  effect  due  to  the  heat 
of  friction  may  also  contribute  in  such  cases. 

In  Fig.  25  is  shown  a  specimen  of  cartridge  brass  (approximately 
70  per  cent  copper,  30  per  cent  zinc),  which  has  been  rather 
severely  cold  worked.  This  illustrates  how  the  deformation  or 
change  of  crystal  form  is  brought  about  by  means  of  internal  slips 
or  "faulting"  along  definite  planes  within  the  individual  crystals. 
These  "faults"  persist  in  the  crystals  and  are  revealed  when  the 
metal  is  etched ;  they  are  not  to  be  regarded  necessarily,  however, 
as  discontinuities  within  the  crystals.  As  the  metal  is  more 
severely  worked,  these  slips  become  so  numerous  that  they  fill 


I'iG.  26. — Microstntctvre  of  annealed  cast  bronze 
showing   the   thickness    of   the    surface    layer    of 
X  ioo 


cold-worked  metal. 


A  specimen  of  cast  zinc  bronze  (approximate  composition, 
copper,  88  per  cent;  tin,  10  per  cent;  zinc,  2  per  cent)  was 
turned  in  the  lathe  and  afterwards  heated  at  600°  C  for  2 
hours.  The  distorted  metal  at  the  surface  recrystallized 
under  this  treatment,  the  cast  metal  of  the  center  did  not. 
Etching  reagent,  alcoholic  solution  of  ferric  chloride 


48  Circular  of  the  Bureau  of  Standards 

the  entire  elongated  grain  and  are  no  longer  to  be  observed  with 
certainty. 

The  depth  to  which  a  metal  (cast)  has  been  distorted  by  cold- 
working,  for  example,  in  machining  operations,  can  often  be 
detected  by  making  use  of  the  fact  that  upon  heating  recrystalli- 
zation  of  the  cold-worked  metal  will  occur.  Fig.  26  shows  a 
specimen  of  cast  bronze 17  which  was  turned  to  size  and  shape  in 
the  lathe.  Upon  annealing  a  specimen,  the  depth  to  which  the 
distortion  of  the  material  extended  was  clearly  shown  by  the 
thickness  of  the  recrystallized  layer. 

V.  EFFECTS  OF  STRUCTURE  UPON  PROPERTIES 

The  ultimate  aim  of  any  metallographic  examination  is  to  show 
in  what  manner  and  to  what  extent  the  characteristics  of  the 
material,  particularly  the  mechanical  properties,  are  dependent 
upon  the  particular  features  characterizing  the  structure  of  the 
metal  under  observation.  To  discuss  here  this  phase  of  the  sub- 
ject, even  in  a  manner  only  approximately  complete,  is  im- 
possible. Only  a  few  of  the  most  obvious  effects  of  structure 
upon  the  properties  will  be  mentioned. 

1.  MECHANICAL  PROPERTIES 

It  must  be  borne  in  mind  in  a  discussion  of  this  subject  that  the 
mechanical  properties  of  any  material,  as  expressed  numerically, 
are  more  or  less  dependent  upon  the  method  of  determination 
used,  for  example,  size  and  shape  of  test  specimen,  rate  at  which 
the  stress  is  applied,  etc.  The  structural  features  of  the  material 
are  closely  related  to  the  mechanical  properties,  this  relationship, 
of  course,  being  much  more  apparent  in  the  case  of  the  grosser  or 
macroscopic  features  than  for  the  very  minute  characteristics, 
as  will  be  evident  in  the  following  examples  which  are  discussed. 
The  previous  treatment,  mechanical  as  well  as  thermal,  also  affects 
the  mechanical  properties,  although  in  this  case  it  may  be  con- 
tended that  the  structural  effects  caused  by  the  previous  treat- 
ments are  largely,  if  not  entirely,  responsible  for  the  changes  noted. 

(a)  HARD  AND  SOFT  CONSTITUENTS 

Many  of  the  alloys  most  useful  from  the  industrial  standpoint 
consist  of  two  or  more  constituents  which  vary  very  widely  in 
their  characteristics.  One  of  the  constituents  is  often  relatively 

17  H.  S.  Rawdon.  Microstructural  Changes  Accompanying  the  Annealing  of  Cast  Bronze,  B.  S.  Tech. 
Papers,  No.  60. 


Structure  and  Properties  of  Metals 


49 


very  soft  and  ductile,  while  a  second  is  hard  and  brittle.  For 
example,  such  a  condition  occurs  in  steels,  in  aluminum  casting 
alloys,  and  in  bronzes,  particularly  those  for  bearing  purposes. 
The  softer  constituent  gives  the  required  ductility,  while  stiffness 
and  strength  are  contributed  by  the  harder  one,  which  is  dissemi- 
nated throughout  the  soft  matrix.  The  relative  proportions  of 
the  two  determine  the  properties  of  the  alloys  of  the  same  general 
series  which  differ  among  themselves  in  their  percentage  composi- 
tion. 

Fig.  27  shows  a  specimen  of  cast  zinc  bronze  (approximately 
88  per  cent  copper,  10  per  cent  tin,  and  2  per  cent  zinc)  which 
was  stressed  in  tension  un- 
til fracture  occurred.    The 
soft    ductile    copper-rich 
matrix    easily   adapted 
itself  to  the  applied  load- 
ing, the  hard,  brittle  tin- 
rich  constituent  was 
shattered  and  broken,  as 

m'^Kat  »..•»:•  „ 


',,.•.  1 

FIG.  27. — Microstructure    of    cast    zinc    bronze 
which  has  been  stressed  in  tension.     X  250 

Note  the  parallel  black  lines  in  the  light-colored  constit- 
uents. These  are  cracks  which  formed  in  the  brittle  con- 
stituent at  right  angles  to  the  direction  in  which  the  stress 
was  applied.  Approximate  composition  of  alloys,  copper, 
88  per  cent;  tin,  10  per  cent;  zinc,  2  per  cent.  Etching 
reagent,  concentrated  ammonium  hydroxide 


shown,  when  stressed  suf- 
ficiently. Similar  cases 
may  be  noted  in  other 
alloys.  Fig.  28  shows  a 
portion  of  a  test  specimen 
of  an  aluminum-casting 
alloy  broken  in  tension. 
The  hard  constituent, 
consisting  of  a  compound 
of  aluminum  and  copper 
(CuAl2),  was  sufficient  in  amount  to  form  a  continuous  network 
throughout  the  alloy.  The  course  or  path  of  the  fracture  of  a  test 
specimen  was  determined  by  this  network,  as  is  shown  in  Fig.  28; 
thus  the  results  of  a  tension  test  of  such  a  material  depend  pri- 
marily upon  the  amount  and  the  properties  of  this  constituent. 

(6)  SOFT  DUCTILE  CONSTITUENTS 

Copper  and  lead  do  not  alloy  with  each  other  in  the  sense  that 
most  metals  do;  neither  solid  solutions  nor  definite  compounds 
of  the  two  are  formed.  An  "alloy"  of  these  metals  consists  only 
of  a  mechanical  mixture  of  the  two  metals,  the  intimacy  of  the 
mix  depending  largely  upon  the  care  used  in  preparation  and  the 

1105F00— 22 4 


50  Circular  of  the  Bureau  of  Standards 

skill  of  the  operator.  The  "alloy"  may  be  considered,  for  con- 
venience, as  a  copper  sponge,  the  interstices  of  which  are  filled 
with  globules  of  lead,  as  is  shown  in  Fig.  29. 

Although  both  copper  and  lead  when  reasonably  pure  are 
highly  ductile,  the  mixture  of  the  two  behaves  in  a  rather  anoma- 
lous manner  when  tested.  The  behavior  of  the  material  when 
stressed  in  tension  is  somewhat  as  might  be  expected.  It  is 
somewhat  ductile,  but  is  decidedly  inferior  to  metallic  copper  in 
its  properties.  Thus  a  tension  test  of  an  alloy  consisting  essen- 
tially of  40  per  cent  lead  and  60  per  cent  copper  gave  the  follow- 


FIG.  28. — ^licrostructure  of  cast  aluminum  alloy  showing  the  "path"  of  the  fracture  pro- 
duced by  a  tensile  stress.     X  loo 

The  alloy  was  of  the  following  approximate  composition:  Copper,  1.8  per  cent;  magnesium,  1.7  per 
cent;  manganese,  1.2  per  cent;  aluminum,  remainder.  Such  an  alloy  consists  of  a  framework  of  a  hard 
constituent  embedded  in  a  much  softer  matrix.  Note  that  the  "path"  of  the  fracture  in  tension  was 
determined  largely  hy  the  framework  of  the  hard  constituent.  This  has  been  indicated  by  arrows. 
Etching  reagent,  o.i  per  cent  solution  of  sodium  hydroxide 

ing  results:  Ultimate  strength  in  tension,  10  650  pounds  per  square 
inch;  elongation  in  2  inches,  8.5  per  cent;  and  reduction  of  area, 
7.5  per  cent.  The  continuity  of  the  copper  matrix  is  so  broken 
up  and  weakened  by  the  inclosed  globules  of  lead,  which  of  course 
are  of  very  low  tensile  strength,  that  the  resulting  tensile  proper- 
ties are  correspondingly  lowered.  The  properties  measured  are 
essentially  those  of  the  copper  sponge,  and  the  properties  of  any 
particular  specimen  are  inferior  to  those  of  a  specimen  containing 
the  same  amount  of  copper  in  the  form  of  a  solid,  but  smaller,  rod. 
The  appearance  of  the  fractured  specimen  of  the  "alloy"  when 
tested  in  compression  is  shown  in  Fig.  29  c.  Although  each  of 


Structure  and  Properties  of  Metals 


51 


the  two  constituents  is  decidedly  ductile  under  compression,  the 
mixture  of  the  two  behaves  in  a  manner  characteristic  of  a  brittle 
material.  Instead  of  flattening  to  any  appreciable  extent,  the 


FIG.  29. — Microstructure  of  copper-lead  alloys  and  appearance  of  a  specimen  of  the  same 

after  testing  in  compression 

"Alloys"  of  copper  and  lead  consist  simply  of  a  mechanical  mixture  of  the  two  metals.  Note  the 
black  spots  and  network  which  indicate  the  spaces  initially  filled  with  globules  of  lead,  traces  of  which 
still  remain  in  place  after  the  polishing  of  the  specimen.  The  samples  were  slightly  etched  with  concen- 
trated nitric  acid  for  developing  the  structure,  (a)  Alloy  of  approximate  composition,  copper,  77  per 
cent;  lead,  23  per  cent.  X  100.  (6)  Alloy  of  approximate  composition,  copper,  60  per  cent;  lead,  40  per 
cent.  X  100.  (c)  Compression  specimen  of  alloy  b  after  test.  X  i.  Note  that  the  specimen  sheared 
when  compressed  sufficiently  in  a  manner  considered  characteristic  of  brittle  alloys  although  each  of  the 
two  component  metals,  copper  and  lead,  is  very  ductile  under  compression 

specimen  shears  in  a  manner  such  as  is  expected,  for  example, 
in  cast  iron.  The  inclosed  globules  of  the  lead  undoubtedly  con- 
tribute largely  to  the  failure  of  the  specimen  in  the  manner  shown, 


52  Circular  of  the  Bureau  of  Standards 

by  their  action  as  a  "lubricant."  The  ultimate  strength  in  com- 
pression of  lead  is  very  much  lower  than  that  of  copper;  thus 
the  lead  yields  under  the  applied  loading  and  "flows"  long  before 
the  copper  is  stressed  to  a  degree  which  would  cause  appreciable 
deformation. 


FIG.  30. — Characteristic  fractures  of  gun   steel    tested  in    tension  across  the  grain   of  the 
metal,  together  with  structure  of  the  same 

Note  the  lack  of  elongation  in  the  specimen  b  after  test.  The  handed  structure  shown  in  c  is  largely 
responsible  for  this,  (a)  Face  of  the  fracture  of  the  tension  specimen  shown  in  side  view  in  b,  slightly  less 
than  X  2;  (c)  longitudinal  medial  section  of  b,  etched  with  aqueous  solution  of  copper  ammonium  chloride, 
slightly  less  than  X  2;  (d)  Microstructure  of  specimen  b,  X  500.  Etching  reagent,  2  per  cent  alcoholic  nitric 
acid 

(c)  ORIENTATION  OF  TEST  SPECIMEN  WITH  RESPECT  TO  MATERIAL  TESTED 

It  has  been  shown  in  a  previous  section  that  the  mechanical 
working  which  is  necessary  for  forming  a  metal  after  casting 
affects  the  structure  to  a  very  marked  extent.  The  worked 
material  has  a  more  or  less  "fibrous"  structure,  depending  largely 
upon  variations  in  composition  across  a  section  of  the  ingot  used 


Structure  and  Properties  of  Metals  53 

and  particularly  upon  the  various  inclusions  within  the  metal. 
After  the  working  of  the  metal  these  are  arranged  in  rather 
definite  lines,  the  course  of  which  is  determined  by  the  shaping  of 
each  particular  piece. 

It  is  evident  that  the  mechanical  properties  when  measured 
across  a  laminated  or  fibrous  material  will  be  quite  different  from 
those  of  the  same  material,  the  test  specimen  of  which  was  cut 


m 


~r 


FIG.  31. — Characteristic  appearance  of  "flakes"  in  gun  steel  revealed  by  tensile  fractures 
and  macroscopic  appearance  of  flaky  steel 

Note  the  white  areas,  a  and  b.  These  correspond  to  the  defects  (discontinuities)  of  c.  (a)  and  (6) 
Fractured  faces  of  tension  specimens  of  flaky  steel.  X  2.  The  "flakes"  have  a  characteristic  silvery 
appearance  and  the  metal  appears  coarsely  crystalline  at  such  spots;  microscopic  examination,  however, 
shows  that  the  grain  is  the  same  across  the  entire  cross  section  of  the  specimen,  (c)  Specimen  of  flaky 
gun  steel  deeply  etched  with  concentrated  hydrochloric  acid.  X  i.  The  acid  widens  and  deepens  the 
discontinuities  (flakes) 

parallel  to  the  course  of  the  fibers.  In  the  latter  case,  which  covers 
by  far  the  greater  majority  of  the  test  specimens  used  in  industrial 
testing,  the  mechanical  properties  are  not  seriously  affected.  It 
is  only  when  it  is  specified,  as  is  done  in  some  particular  cases,  for 
example,  gun  forgings,  that  the  test  specimen  shall  be  cut  trans- 
versely to  the  direction  of  working,  that  the  effect  is  marked. 
Fig.  30  shows  the  appearance  of  a  specimen  of  gun  steel  (carbon, 
0.48  per  cent;  manganese,  0.76  per  cent;  nickel,  2.85  per  cent; 


54 


Circular  of  the  Bureau  of  Standards 


sulphur,  0.02  per  cent;  phosphorus,  0.02  per  cent)  broken  in  ten- 
sion, the  following  results  being  obtained :  Yield  point  (by  divider 
method)  79  ooo  lbs/in.2;  ultimate  strength,  83  ooo  lbs./in.2;  reduc- 
tion of  area,  3.5  per  cent;  elongation  in  2  inches,  3.5  per  cent. 
The  very  marked  banded  structure  of  the  material,  which  macro- 
scopic examination  showed  was  caused  by  threads  of  inclusions, 
is  unquestionably  the  reason  for  the  very  low  ductility  shown  by 
the  material.  From  the  microstructure  of  this  material  (Fig.  30  d) 
one  has  reason  to  expect  that  considerable  ductility  would  be 
shown. 

In  Table  2  are  summarized  the  results  obtained  by  testing 
duplicate  transverse  and  longitudinal  specimens  of  the  same  mate- 
rial in  tension.  The  metal  chosen  was  a  type  of  defective  steel, 
encountered  particularly  in  gun  forgings,  which  has  been  desig- 
nated as  "flaky  steel,"  of  a  composition  very  similar  to  that  given 
above.  Agreement  among  metallurgists  as  to  the  origin  of  these 
defects,  "flakes,"  has  not  yet  been  reached.  Their  appearance  as 
revealed  by  a  tension  break  is  shown  in  Fig.  3 1 ;  deep  etching  of 
such  steel  in  concentrated  acid  reveals  their  presence,  as  is  shown 
in  the  same  figure. 

TABLE  2. — Tensile  Properties  of  Flaky  Steel  as  Revealed  by  Transverse  and  by 
Longtitudinal  Test  Specimens 


Specimen 

P-limit 

Yield 
point 

Ultimate 
strength 

Reduc- 
tion of 
area 

Elonga- 
tion in 
2  inches 

Modulus 
of  elas- 
ticity 

Transverse  

Lbs./in.2 
53500 

Lbs./in.= 
56100 

Lbs./in.2 
59200 

Per  cent 
1.5 

Per  cent 
1.5 

Lbs./in.2 
29000000 

Do 

65  000 

67  000 

92  950 

5.  0 

3.5 

29  000  000 

Longitudinal 

62  500 

65  000 

106  500 

52  0 

26  5 

29  500  000 

Do  

62500 

65000 

106  850 

50.5 

26.0 

29500000 

It  is  very  evident  from  the  results  given  that  the  relation  which 
the  test  specimen  bears  to  the  parent  material  affects  the  meas- 
ured mechanical  properties  of  the  material  to  a  marked  degree. 
Defective  material  of  the  type  used  when  tested  in  the  ordinary 
manner  shows  apparently  very  superior  properties,  particularly 
in  ductility.  However,  when  a  specimen  cut  transversely  from 
the  material  is  tested,  it  behaves  very  differently  and  breaks  with 
practically  no  ductility.  The  elastic  properties  are  not  seriously 
affected,  however,  even  when  the  internal  defects  are  sufficient 
to  reduce  the  ultimate  strength  of  the  material  to  approximately 
only  50  per  cent  of  the  normal  value.  When  such  material  is 


Structure  and  Properties  of  Metals 


55 

subjected  to  some  of  the  dynamic  methods  of  testing,  impact, 
fatigue,  etc.,  the  difference  in  the  results  obtained  for  the  trans- 
verse specimen  is  usually  even  more  marked  than,  the  results 
given  above  for  the  tension  test. 

(d)  COARSELY   GRAINED 
METALS 

It  has  been  previ- 
ously suggested  that 
the    grain    size  of    a 
metal  has   a  pro- 
nounced effect  upon 
many  of  the  proper- 
ties of  the  material. 
Coarsely  grained  met- 
als are  quite  univer- 
sally   regarded    with 
disfavor,    although 
there   appears  to  be 
no  evidence  at  hand 
to    demonstrate   the 
unsuitability  of  such 
material    for    many 
purposes.     The  brit- 
tleness  usually  attrib- 
uted   to   large   grain       V 
size  is  not  very  well      FlG.  32^Macrostructu 
revealed  by  a  tension 
test,  at  least  as  ordi- 
narily carried  out. 
Fig.  32  shows  a  sec- 
tion   of     coarsely 
grained  tension  speci- 
mens of  cast  bronze. 


of  tension  specimens  of  cast 
zinc  bronze  showing  the  relation  of  the  fracture  produced 
to  the  crystalline  structure.  X  3 

Note  the  very  large  grain  size  of  both  specimens,  (a)  and  (6) 
Longitudinal  medial  sections  through  the  fractured  ends  of  two 
tension  specimens  of  cast  bronze  (approximate  composition,  cop- 
per, 88  per  cent;  tin,  10  per  cent;  zinc,  t  per  cent).  In  both  speci- 
mens the  path  of  the  fracture  was  transcrystalline.  The  specimens 
were  etched  with  alcoholic  solution  of  ferric  chloride  and  then  cov- 
ered with  a  coating  of  shellac 


The  structure  indicates  that  the  mechanical  properties  were 
determined  to  a  very  large  degree  by  the  fact  that  the  entire 
cross-sectional  area  at  the  point  at  which  the  fracture  occurred 
comprised  only  a  few  large  grains.  There  was  nothing  how- 
ever, to  indicate  this  until  the  specimen  was  sectioned  and  its 
structure  examined.  A  shock  or  impact  test  reveals  the  effect  of 
coarse  grain  in  a  much  more  striking  manner.  Fig.  33  shows  the 
appearance  of  two  specimens  of  the  same  steel  after  testing  in  a 


Circular  of  the  Bureau  of  Standards 


FIG.  33. — Appearance  of  specimens  of  low-carbon  steel  uith  different,  thermal  treatments 
showing  the  relation  of  impact  properties  to  the  microstructure  of  the  material 

(a)  and  (6)  Fractured  face  and  side  view  of  specimen  of  low-carbon  steel  broken  by  the  Fremont  impact 
test  of  a  falling  weight.  X  iH.  (c)  and  (d)  Fractured  face  and  side  view  of  a  second  specimen  of  the  same 
material  tested  in  same  way.  X  i%.  (e)  Microstructure  of  specimen  of  0-6.  X  100.  The  steel  has  been 
rendered  coarsely  grained  evidently  by  overheating  in  the  annealing  process.  (/)  Microstructure  of  speci- 
men c-d.  X  100.  Etching  reagent,  2  per  cent  alcoholic  solution  of  nitric  acid,  (g)  Microstructure  of  soft 
iron  wire  (very  low-carbon  steel)  which  was  as  "brittle  as  glass"  when  an  attempt  was  made  to  bend  it 
at  the  temperature  of  liquid  air.  X  100.  (A)  Microstructure  of  a  wire  of  similar  composition  which  proved 
to  be  very  tough  at  the  temperature  of  liquid  air  and  withstood  several  complete  bends,  180°,  before  break- 
ing. X  100.  Note  the  difference  in  grain  size  of  the  brittle  material  e  and  g  as  compared  with  similar  metal 
in  the  tough  condition, /and  h 


Structure  and  Properties  of  Metals  57 

Fremont  impact  testing  machine ;  one  of  the  specimens  was  spoiled 
evidently  in  the  annealing  process;  the  second,  on  the  other  hand, 
showed  exceptionally  superior  qualities.  The  microstructure  sug- 
gests that  the  superior  shock-resisting  properties  were  undoubtedly 
produced  by  quenching  the  steel  from  a  high  temperature,  prob- 
ably above  that  of  the  A3  transformation.  The  entire  lack  of 
ductility  of  the  coarsely  grained  specimen  as  compared  with  the 
superior  shock-resisting  qualities  of  the  same  steel  when  properly 
treated  affords  striking  evidence  of  the  influence  of  grain  size  upon 
mechanical  properties.  Fig.  33,  e  and  /,  illustrates  the  fact  that 
at  extremely  low  temperatures  the  grain  size  of  metals  is  a  fac- 
tor of  even  greater  importance  in  determining  the  mechanical 
properties  than  at  ordinary  temperature. 

(e)  PHYSICAL  STATE  OF  MICROSCOPIC  CONSTITUENTS 

The  relative  size,  arrangement,  and  method  of  distribution  of 
the  various  constituents  which  make  up  the  structure  of  an  alloy 
bear  a  close  relationship  to  the  various  properties  of  the  alloy. 
This  is  best  noted  in  a  binary  alloy,  for  example,  eutectoid  carbon 
steel,  different  specimens  of  which  have  been  subjected  to  various 
thermal  treatments  with  the  express  purpose  of  producing  the 
variations  in  the  microstructural  features  suggested  above.  Of 
course  precautions  must  be  taken  that  no  phase  changes  in  the 
alloy  occur  and  that  it  is  in  stable  equilibrium  throughout,  the 
differences  produced  in  the  structure  being  physical  ones  only. 

Considerable  investigational  work  has  been  done  to  show  how 
the  mechanical  properties  of  carbon  steels,  particularly  those  of 
eutectoid  composition,  vary  with  the  physical  state  of  the  pearlite, 
the  steel  being  in  the  softened  state  throughout  and  the  pearlite 
ranging  from  the  lamellar  type  through  various  stages  to  the  com- 
pletely "  divorced  "  or  spheroidized  condition.  The  work  of  Hane- 
mann  18  and  that  of  Howe  19  may  be  cited  as  illustrative  of  this. 

The  influence  of  the  physical  state  of  the  pearlite  in  steel  upon 
the  properties  is  well  shown  by  a  study  of  the  magnetic  character- 
istics of  the  same  steel  after  various  treatments.20  The  various 
specimens  of  the  steel  (carbon  0.85  per  cent;  manganese,  0.28  per 
cent)  were  cooled  from  the  same  temperature  (800°  C)  at  rates  so 
chosen  that  the  structure  of  the  different  specimens  varied  from  a 

18  H.  Hanemann  and  F.  Morawe,  Uber  den  kornigen  Perlit  und  seiner  Bedeutung  Kir  die  Warmebehand- 
Inng  des  Stahls,  Stahl  und  Eisen,  33,  Part  2,  p.  135°:  1913. 

19  H.  M.  Howe  and  A.  G.  L,evy,  "Notes  on  pearlite,"  J.    Iron  and  Steel  Inst.,  94,  p.  210;  1916. 
80  C.  Nusbaum  and  W.  I,.  Cheney,  B.  S.  Sci.  Papers,  No.  408;  1920. 


58  Circular  of  the  Bureau  of  Standards 

fine  sorbitic  condition  to  a  "divorced"  or  spheroidized  pearlite  as 
shown  in  Fig.  34.  The  magnetic  properties  of  the  corresponding 
specimens  are  given  graphically  in  Figs.  35  and  36.  Without  dis- 
cussing here  the  significance  of  the  various  properties  revealed  by 
the  magnetic  tests,  it  will  be  evident  that  the  properties  of  the 


FIG.  34. — Microstructure  of  eutectoid  carbon  steel  (0.85  per  cent  carbon)  showing  the 
effect  of  rate  of  cooling  upon  the  physical  state  of  the  pearlite.     X  500 

Note  how  the  lamellae  constituting  the  pearlite  became  much  thicker  and  more  pronounced  as  the  an- 
nealing progressed.  The  physical  properties  of  the  material  also  changed  in  corresponding  manner.  The 
specimens  were  heated  to  800°  C  and  cooled  as  follows:  (a)  Cooled  in  air,  the  material  consists  largely  of 
sorbite;  (6)  cooled  in  lime,  the  material  contains  patches  of  fine  lamellar  pearlite  and  some  sorbite;  (c) 
cooled  in  furnace,  coarse  lamellar  pearlite  with  some  spheroidizing  of  the  pearlite  has  resulted;  (d)  cooled 
in  furnace  at  a  much  slower  rate  than  in  c,  the  pearlite  was  largely  spheriodized  or  "divorced. "  Etching 
reagent,  5  per  cent  alcoholic  solution  of  picric  acid 


Structure  and  Properties  of  Metals 


59 


Magnet/zing  Ferce _, 

FIG.  35. — Magnetic  properties  (induction  vs.  magnetizing  force)of  eutectoid  carbon  steel 
after  different  annealing  treatments 


/r?a  force ,  ji/be/*fe  per  err? 

FlG.  36. — Magnetic  properties  (permeability  vs.  magnetizing  force)  of  eutectoid  carbon 
steel  after  different  annealing  treatments,  replotted from  Fig.  35 

steel  are  affected  to  a  marked  degree  by  the  changes  which  have 
been  brought  about  in  the  physical  state  of  the  pearlite.  Corre- 
sponding differences  in  the  mechanical  properties  would  also  be 
found  upon  testing,  although  perhaps  of  not  so  great  a  magnitude 
as  in  the  magnetic  properties,  since  the  magnetic  tests  are  much 


O ,  spec/mend;  9,  B;  9.C, 
®,D.  See 


6o 


Circular  of  the  Bureau  of  Standards 


more  sensitive  than  the  ordinary  mechanical  ones  and  often  reveal 
changes  which  are  detectable  by  almost  no  other  means. 

2.  CHEMICAL  PROPERTIES 

The  chemical  property  of  metals  and  alloys  which  is  probably 
most  important  industrially  is  that  designated  by  the  rather  loose 
term  of  "solubility."  Upon  this  property  depends  the  etching  of 
metallographic  specimens,  the  coloring  of  metallic  surfaces,  the 
corrodibility  of  materials  under  service  conditions,  and  often,  by 


£« 

/ 

\ 

«/ 

^r 

/ 

\ 

4- 

/ 

\ 

3     • 

L 

\ 

\ 

? 

/ 

\ 

X 

/ 

/ 

0ne  per  cent  carfor?  steel 
hardened  and  f  hen  tempered 

10                400                3~00                £00*  G 

X 

0                  /t 

/ 

•^           % 

JO                  3 

FIG.  37. — Variation  in  the  solubility  of  I  per  cent  carbon  steel  in  dilute  sulphuric  acid, 
after  hardening  and  tempering  to  different  temperatures.    (Hanemann) 

the  selective  corrosion  of  certain  constituents,  the  complete 
deterioration  of  the  entire  alloy  in  service.  This  property  of  an 
alloy  is  often  influenced  to  a  marked  degree  by  the  structure,  and 
the  following  examples  are  cited  as  typical  of  this  effect  of  struc- 
ture upon  properties. 

(a)  ETCHING 

The  etching  of  the  polished  surface  for  revealing  the  micro- 
structure  is  dependent  primarily  upon  the  relative  solubility  of  the 
constituents  comprising  the  alloy.  In  case  of  a  one-constituent 
alloy  the  difference  in  the  rate  of  solubility  of  the  various  crystals, 


Structure  and  Properties  of  Metals  61 

which  in  turn  results  from  the  orientation  of  the  sectioning  plane 
relative  to  the  inner  structure  of  the  different  crystals  comprising 
the  alloy  cut  by  this  plane,  is  the  fundamental  reason  for  the  pro- 
duction of  the  etch  pattern.  It  is  a  well-established  fact  that  the 
solubility  of  a  crystalline  material  varies  considerably  when 
measured  in  different  directions  in  one  and  the  same  crystal. 

The  constituents  of  binary  as  well  as  of  more  complex  alloys 
practically  always  differ  in  their  electrochemical  properties,  one 
being  electronegative  to  the  others.  Consequently  such  a  con- 
stituent will  be  attacked  by  an  etching  reagent  in  preference  to 
the  others,  which  remains  bright  and  often  stand  out  in  slight 
relief  above  the  surface.  The  microconstituents  behave  then  very 
similarly  to  two  dissimilar  metals  in  contact  with  each  other  in 
an  electrolyte,  as  is  illustrated  by  steel  coated  with  copper,  nickel, 
or  tin,  which  are  all  electropositive  toward  iron  (or  steel)  and 
which  thus  accelerate  the  corrosion  of  the  iron  base  of  the  coated 
sheet  after  it  has  been  exposed  by  abrasion  or  in  other  ways.  A 
zinc  coating  on  the  other  hand  being  electronegative  to  iron,  the 
two  being  in  contact  within  an  electrolyte,  protects  the  exposed 
spot  of  iron  for  a  considerable  time  at  the  expense  of  itself  until  the 
exposed  area  becomes  too  large. 


FIG.  38.— Appearance  of  a  I  per  cent  steel  rod  hardened,  differentially  tempered,  and  then 
immersed  in  dilute  sulphuric  acid.     X  I 

The  hardened  rod  was  differentially  tempered  by  heating  the  end  x,  the  other  one  being  kept  cold  by 
water.  Note  the  narrow  ring  encircling  the  rod  where  the  action  of  the  acid  upon  the  material  was  most 
intense 

(6)  SOLUBILITY  OF  TEMPERED  STEELS 

It  is  quite  well  recognized  that  the  solubility  of  steel  varies 
considerable  according  to  the  heat  treatment  which  it  has  pre- 
viously received.  Fig.  37  illustrates  this  variation  of  solubility 
according  to  treatment.21  A  rod  of  high-carbon  steel  (approxi- 
mately i  per  cent  carbon)  was  hardened  by  quenching  in  water 
from  a  temperature  of  765 °  C.  The  hardened  rod  was  differentially 
tempered  by  heating  one  end  to  approximately  850°  C  while  the 
other  was  kept  cool  with  water.  Thus  the  rod  represents,  at 
different  points  along  its  length,  tempering  to  all  temperatures 

«i  H.  Hanemann,  Einfuhrung  in  die  Metallographie  and  Warmebehandlung,  p.  87. 


62  Circular  of  the  Bureau  of  Standards 

between  the  two  extremes.  When  immersed  in  dilute  sulphuric 
acid  (20  per  cent  solution)  for  1 7  hours  the  appearance  shown  in 
Fig.  38  was  produced.  The  material  in  one  of  the  intermediate 
stages  of  tempering  is  the  most  readily  soluble,  rather  than  the 
very  hard  or  the  very  soft  portions. 

Considerable  attention  has  been  given  to  this  property  of  tem- 
pered steels  by  foreign  metallurgists,  and  it  has  been  shown,  as 
is  illustrated  by  Fig.  37,  that  the  rate  of  solubility  can  be  used 
as  an  index  of  the  tempering  a  specimen  of  steel  has  received. 
Maximum  solubility  corresponds  to  a  tempering  at  400°  C.  A 
special  name,  "  osmondite,"  has  been  given  to  steel  in  this  particu- 
lar condition,  on  account  of  its  characteristic  properties. 

From  Figs.  37  and  38  it  is  evident  that  the  rate  of  solution  is 
profoundly  affected  by  the  structure  of  the  steel.  In  the  form 
of  the  solid  solution  the  material  is  the  least  soluble.  As  the 
degree  of  tempering  is  increased  and  the  carbide  held  in  solid 
solution  is  progressively  precipitated,  the  rate  progressively 
increases.  The  maximum  solubility  occurs  in  the  troostitic  steel 
after  the  simple  solid  solution  has  been  changed  by  tempering 
into  a  state  of  agglomeration  resembling  that  of  an  emulsion. 
The  material  in  this  condition  is  not  resolvable  under  the  micro- 
scope into  its  constituent  parts.  As  the  tempering  is  continued, 
progressively  higher  temperatures  being  used,  the  ultramicroscopic 
particles  increase  in  size  until  finally  the  ordinary  microscopic 
examination  reveals  them.  This  increase  in  the  size  of  particles 
of  the  constituents  is  accompanied  by  a  decreased  solubility, 
although  the  fully  annealed  specimen  is  somewhat  more  soluble 
than  is  the  material  in  its  initial  or  fully  hardened  state. 

(c)  CORROSION 

This  term  when  strictly  used  refers  to  the  tendency  of  metals  to 
revert  to  the  stable  form  in  which  they  occur  in  nature;  that  is, 
the  oxide.  The  subject  is  so  broad  and  the  contributing  causes 
so  many  and  so  varied  that  considerable  differences  of  opinion  are 
held  as  to  the  mechanism  by  which  the  process  of  corrosion  is 
brought  about.  Only  a  brief  mention  will  be  made  here  of  how 
structural  features  may  aid  in  the  process. 

In  general,  the  corrosive  attack  of  a  metal  is  more  pronounced 
in  the  direction  of  the  "fibers"  than  across  them.  This  is  most 
marked  in  case  of  accelerated  corrosion  by  exposure  to  sea  air, 
sea  water,  or  similar  conditions,  and  is  found  in  all  types  of  readily 
corrodible  alloys  if  in  the  wrought  state.  This  may  be  attributed 


Structure  and  Properties  of  Metals 


FIG.  39. — Appearance  of  a  corroded  brass  sheet  together  with  the  microstructure  of  the 
unchanged  and  of  the  corroded  metal 

Note  how  brittle  the  sheet  a  was  after  corrosion.  This  was  the  result  of  a  selective  corrosive  attack  of 
one  of  the  two  microconstituents  which  make  up  the  alloy,  (a)  The  brass  sheet,  approximate  composi- 
tion, copper,  60  per  cent;  zinc,  40  per  cent,  became  so  brittle  after  corrosion  in  sea  water  that  it  could  be 
broken  into  fragments  with  the  fingers  as  shown.  X  i.  (6)  Microstructure  of  Muntz  metal  showing  the 
two  constituents  a  and  ft.  X  500.  Etched  with  ammonium  hydroxide  and  hydrogen  peroxide,  (c) 
Microstructure  of  a  corroded  sheet  of  Muntz  metal  after  7  years  exposure  in  sea  water;  the  ft  constituent 
disintegrated  into  a  weak  pulverulent  material  resembling  copper.  X  500.  The  specimen  was  etched 
with  ammonium  hydroxide  and  oxidized  in  the  air 


Circular  of  the  Bureau  of  Standards 


FIG.  40. — Appearance  of  lead  after  accelerated  corrosion  and  of  commercial  lead  which 
corroded  in  service 

Note  in  e  how  the  bond  between  the  crystals  of  lead  may  be  so  weakened  under  certain  conditions  as  to 
permit  the  metal  to  be  crumbled  as  shown  in  6.  (a)  Surface  appearance  of  a  sheet  of  high-grade  lead  (99.99 
per  cent)  after  immersion  of  24  days  in  a  solution  of  lead  acetate  (N)  and  nitric  acid  (0.8  N)  and  bent  at  a 
slight  angle.  Anintercrystalline  brittleness  resulted  from  the  corrosive  attack;  X  5.  (6)  Crystals  which 
were  detached  from  a  sheet  of  commercial  lead  (lead,  99.72  per  cent;  antimony,  0.07  per  cent ;  iron,  0.02  per 
cent;  tin,  0.14  per  cent)  immersed  for  4  days  in  an  acidified  solution  of  lead  acetate  (400  g  lead  acetate,  100 
cm3  nitric  acid,  1000  cm3  water).  X  8.  The  corrosion  of  the  lead  was  intercrystalline  in  nature,  (c)  Some 
of  the  crystals  of  6  flattened  out.  X  8.  Each  crystal  retained  the  characteristic  malleability  of  lead, 
although  the  sheet  as  a  whole  was  brittle,  (d)  Appearance  of  a  lead  cable  sheathing  which  corroded  in 
service,  approximate  composition:  tin,  1.09  per  cent;lead  98.3  percent.  X  iK.  (e)  Cross  section  of  speci- 
men d,  unetched.  The  upper  edge  of  the  micrograph  coincided  with  the  outer  surface  of  the  sheath.  The 
corrosive  attack  of  the  metal  was  intercrystalline  i  n  its  nature.  X  50 


Structure  and  Properties  of  Metals  65 

largely  to  the  mechanical  effect  of  inclusion  streaks  by  affording 
lodgment  for  moisture,  so  that  the  attack  at  such  points  is  accel- 
erated and  access  to  the  interior  more  readily  gained  along  the 
line  of  the  inclusion.  Similar  instances  have  been  noted  in 
wrought-aluminum  alloys,  in  which  it  appears  that  the  difference 
in  the  electrochemical  properties  of  streaks  of  the  metal  in  various 
stages  of  cold  working  is  responsible  in  a  large  degree  for  the 
fibrous  appearance  of  the  corroded  ends  of  wrought  rods. 

Brass  of  the  type  termed  Muntz  metal,  approximately  60  per 
cent  copper  and  40  per  cent  zinc,  exemplifies  well  the  specific 
effect  of  a  metallographic  constituent  upon  the  corrosion  of  a 
metallic  material.  Such  a  brass  has  a  duplex  structure  as  is 
shown  in  Fig.  39  b,  one  constituent,  the  a,  being  much  richer  in 
copper  than  is  the  second,  or  ft.  The  zinc-rich  constituent  is  quite 
readily  attacked  by  sea  water,  the  difference  in  the  electrochemical 
potential  of  the  two,  a  and  ft,  contributing  to  bring  about  the 
difference  in  the  resistance  of  the  two  to  the  action  of  the  elec- 
trolyte. The  zinc  from  the  ft  is  leached  out,  and  a  spongy  mass 
resembling  copper  remains,  filling  the  spaces  previously  occupied 
by  the  ft  (Fig.  39  c).  Thus  the  alloy  is  converted  into  a  weak 
brittle  mass  consisting  of  a  sponge  of  the  more  or  less  attacked 
a  constituent  and  the  pulverulent  material  resulting  from  the  dis- 
integration of  the  ft.  Material  of  this  kind  in  the  form  of  sheets 
often  becomes  so  brittle  that  it  can  be  readily  broken  into  small 
fragments  in  the  fingers  (Fig.  39  a). 

A  soft  ductile  metal  like  lead  may  under  some  conditions  of 
accelerated  corrosion  become  so  brittle  that  it  can  be  crumbled 
to  powder  in  the  fingers.  This  is  most  apt  to  occur  if  the  lead 
is  somewhat  impure.  Fig.  40  shows  a  section  of  a  lead  cable 
sheath  which  by  electrolytic  corrosion  became  so  brittle  that  it 
could  be  reduced  to  a  granular  powder.  Each  grain,  however, 
retained  its  initial  ductility  and  other  characteristic  properties  of 
lead.  The  corrosive  attack  of  the  metal  was  essentially  inter- 
crystalline  in  character  (Fig.  40  d)  which  was,  in  al1  probability, 
due  largely  to  the  impurity  contained  by  the  metal  (1.09  per 
cent  tin) ,  although  it  has  been  shown 22  that  lead  of  exceptionably 
high  purity  (99.99  per  cent)  is  subject  to  intercrystalline  brittle- 
ness  under  certain  conditions.  In  the  corroded  lead  illustrated 
in  Fig.  40  most  of  the  impurities  which  are  present  exist  as 

11  H.  S.  Rawdon,  Intercrystalline  Brittleness  of  Lead,  B.  S.  Sci.  Papers.  No.  377;  1930. 
110580°— 22— 5 


66 


Circular  of  the  Bureau  of  Standards 


tiny  lodgments,  sometimes  as  a  eutectic,  between  the  crystals  of 
lead;  thus  the  corrosive  attack  is  localized  to  a  large  extent, 
and  the  metal  as  a  whole  shows  intercrystalline  brittleness. 

GO  VARIATION  IN  COMPOSITION  THROUGHOUT  AN  ALLOY 

In  some  alloys  the  composition  varies  at  times  to  such  an 
extent  throughout  the  body  of  the  alloy  that  the  properties, 
both  chemical  and  physical,  are  far  from  being  uniform  for  the 
material.  Examples  of  this  have  already  been  spoken  of  (Sec. 
Ill,  2).  Segregation,  as  in  steel,  caused  by  the  selective  process 
____________________________________  °f  freezing,  is  responsi- 
ble for  many  such 
marked  cases  of  chemi- 
cal unhomogeneity. 
Fig.  41  illustrates  a 
marked  case  of  such  un- 
homogeneity i  n  lead- 
___ ^  wm  ,  ,„  antimony  alloys  because 

of  the  difference  of  den- 
<=*  £> 


FIG.  41. — Chemical  unhomogeneity  of  lead-antimony  al- 
loys caused  by  "  liquation ' '  during  slow  cooling.  X  i 
Note  the  layer  of  the  white  constituent  in  each  specimen 


sity  of  the  constituents 
of  the  alloy,  the  cause  in 
this  case  being  usually 


the  ingot  during  solidification.  This  illustrates  one  of  the  diffi- 
culties encountered  in  preparing  bearing  metal  alloys,  (a)  Lead- 
antimony  alloy  containing  approximately  15  per  cent  antimony. 
The  greater  part  of  the  small  ingot  is  composed  of  the  eutectic 
which  contains  13  per  cent  antimony,  the  excess  antimony  rose 
to  the  top  of  the  molten  metal  because  of  its  lower  density.  (6) 
Similar  alloy  containing  22  per  cent  antimony.  The  specimens 
were  etched  with  dilute  hydrochloric  acid  and  then  slightly 


oe       e    ayer  o        e  we  consuen      n  eac     specmen.  ..  .. 

This  consists  of  antimony  crystals  which  collected  at  the  top  of       S  p  O  K  C  n     OI      aS 

tion."  It  is  evident 
without  further  discus- 
sion that  a  specimen  cut 
from  the  lower  portion 

of     each    of       the     ingOtS 

repoiished  w^  snow  properties 

which  will  differ  markedly  from  those  of  a  specimen  taken  from 
near  the  top  of  the  ingot. 

VI.  APPLICATIONS  OF  THE  MICROSCOPY  OF  METALS 

The  most  important  industrial  application  of  the  microscopy 
of  metals  is  undoubtedly  its  use  in  connection  with  the  heat- 
treatment  of  steels.  Not  only  is  it  used  extensively  as  a  routine 
method  of  examination  of  heat-treated  stock  to  make  certain 
that  the  prescribed  treatment  has  been  carried  out  and  also  to 
show  wherein  lies  the  fault  when  the  material  after  treatment 
does  not  have  the  expected  mechanical  properties,  but  it  is  of 


Structure  and  Properties  of  Metals  67 

inestimable  service  in  specifying  a  proper  heat  treatment  for  new 
types  of  steels.  A  second  important  application  of  the  method 
is  as  a  supplement  to  chemical  analysis.  It  is  believed,  however, 
that  the  description  of  the  various  applications  can  best  be  given 
by  means  of  short  references  to  a  few  typical  cases  chosen  from 
numerous  materials  examined  at  the  Bureau  of  Standards  than 
by  a  longer  and  more  general  discussion  such  as  exists  already 
in  some  of  the  textbooks  on  the  subject  of  metallography. 

1.  RELATION  TO  HEAT  TREATMENT 

As  has  been  previously  stated  (Sec.  V,  i,  d)  the  grain  size  of 
a  metal  is  of  very  considerable  importance  in  affecting  the  prop- 
erties of  the  material.  In  most  cases  the  microscope  should  be 
depended  upon  for  revealing  accurately  the  relative  grain  size 
of  metallic  materials,  although  considerable  information  can  be 
gained  by  examining  the  fracture  of  broken  test  specimens,  and 
it  is  claimed  by  some  that  for  some  materials,  particularly  alloy 
steels,  the  examination  of  the  fracture  is  the  only  sure  way  of 
arriving  at  this  information.  If  a  numerical  expression  of  the 
grain  size  is  desired,  the  method  recommended  by  the  American 
Society  for  Testing  Materials  may  be  used.23 

Fig.  22  shows  a  medium-carbon  steel  (carbon  0.46  per  cent) 
which  has  been  ' '  overheated ' ' — it  was  maintained  for  six  hours  at 
1110°  C.  The  steel  was  not  permanently  injured,  however,  and 
it  was  "restored"  by  proper  annealing  as  shown  in  Fig.  22  c. 
However,  material  which  is  heated  at  so  high  a  temperature  that 
it  is  properly  described  as  ' '  burnt ' '  is  useless ;  it  can  not  be 
restored  and  made  safe  for  use  by  any  treatment  now  known  short 
of  remelting.  The  heating  of  steel  to  the  temperature  at  which 
incipient  fusion  on  the  surface  occurs  does  not,  however,  always 
necessarily  entail  "burning"  of  the  material.  For  some  of  the 
special  alloy  steels  used  for  high-speed  cutting  purposes,  the  heat 
treatment  recommended  usually  includes  "heating  to  incipient 
fusion."  The  restoration  of  the  material  after  overheating 
depends  upon  the  phase  change  which  occurs  in  the  alloy  at  its 
critical  temperature.  Such  a  simple  means  for  removing  the 
structural  effects  of  overheating  could  not  be  applied  to  a  metal 
such  as  copper,  for  example,  which  exhibits  no  phase  changes  upon 
heating. 

»«  Proc.  Am.  Soc.  for  Testing  Materials,  Specifications  E  2-20,  Micrographs  of  Metals  and  Alloys,  Part 


i; 


68 


Circular  of  the  Bureau  of  Standards 


FIG.  42.  —  Microslruclure  of  high-speed  tool  steel   illustrating  the  effect  upon  structure  of 
tempering  the  hardened  steel  at  different  temperatures.     X  500 


ausencn     s  properes.  pecmen  a,    empere    3 

markings  suggestive  of  the  beginning  of  the  change  into  t 
pered  30  minutes  at  400°  C;  the  martensitic  pattern  has  b 

° 


. 

arg        ggv  gg  o  g     n      the  martensitic  condition,     (f)  Specimen  b,  tem- 

pered 30  minutes  at  400°  C;  the  martensitic  pattern  has  been  fully  developed,     (d)  Specimen  <-,  tempered 
30  minutes  at  600°  C;  although  in  the  troostitic  state,  the  martensitic  pattern  is  still  evident.     (<?)  Specimen 
° 


Structure  and  Properties  of  Metals  69 

A  knowledge  of  the  microstructure  is  very  helpful  in  explaining 
the  peculiar  characteristic  properties  of  high-speed  tool  steels  in 
orcter  to  specify  properly  the  necessary  heat  treatment.24  When 
quenched  from  a  sufficiently  high  temperature  the  material  is,  at 
least  partially,  austenitic  in  structure.  Upon  tempering  at  a 
relatively  low  temperature,  it  is  converted  into  the  martensitic 
state,  with  an  accompanying  gain  in  hardness,  usually  termed 
secondary  hardness.  The  martensitic  state  is  changed  into  the 
troostitic  condition  only  very  slowly,  so  that  the  material  passes 
no  further  than  the  troostitic  state  upon  tempering  at  a  relatively 
high  temperature.  Hence  this  state,  together  with  the  accom- 
panying cutting  properties  (toughness  and  hardness) ,  is  retained 
by  the  material  at  the  high  temperature  which  prevails  in  the  use 
of  such  tools.  Fig.  42  illustrates  some  of  the  characteristic 
structural  features  of  tempered  high-speed  steel. 

High-carbon  steels  maintained  for  a  considerable  time  at  the 
"annealing  temperature"  will  sometimes  show  evidence  in  their 
structure  of  the  formation  of  graphite  at  the  expense  of  the 
cementite.  Such  a  steel  is,  of  course,  useless  for  most  purposes 
and  particularly  so  for  the  purposes  for  which  high-carbon  or  tool 
steel  is  commonly  used.  Fig.  21  shows  the  structure  of  a  tool 
steel  (approximately  i  per  cent  carbon)  which  was  spoiled  in  this 
manner. 

Fig.  43  shows  the  structures  which  may  be  obtained  by  hardening 
a  hypoeutectoid  steel  (carbon,  0.46  per  cent)  by  quenching  from 
different  temperatures.  When  a  temperature  only  slightly  above 
the  A!  transformation  is  used,  considerable  ferrite  exists  inter- 
mixed with  the  martensite,  and  the  material  has  not  been  fully 
hardened.  When  quenched  from  above  the  A,_3  transformation 
temperature,  no  ferrite  remains,  and  a  fine  martensitic  structure 
results.  If  a  still  higher  temperature  is  used,  the  structure  be- 
comes very  coarse,  and  intercrystalline  cracks  often  form  upon 
quenching.  The  time  interval  for  which  the  material  is  maintained 
at  the  temperature  before  quenching  must,  of  course,  be  taken  into 
consideration  in  work  of  this  kind. 

Fig.  44  shows  the  structure  of  a  steel  cylinder,  used  as  a  con- 
tainer for  compressed  gas,  which  exploded  during  use.  The 
specifications  for  this  material  required  that  the  steel  be  tempered 
after  hardening,  the  desired  structure  being  shown  in  Fig.  44  b. 
It  is  evident  from  the  fact  that  the  structure  of  the  defective 


"  H.  Scott,  Relation  of  the  High- Temperature  Treatment  of  High-Speed  Steel  to  Secondary  Hardening 
and  Red  Hardness,  B.  S.  Sci.  Papers,  No.  395;  1920. 


7o 


Circular  of  the  Bureau  of  Standards 


•I 


FIG.  43. — Microstructure  of  medium  carbon  steel  after  different  hardening  treatments 

Note  the  light  colored  constituent  in  a.  This  indicates  that  the  steel  was  not  fully  hardened  by  the  treat- 
ment given.  The  large  crystals  in  c,  together  with  the  fine  quenching  cracks,  show  that  the  hardened 
treatment  was  too  severe,  (a)  Specimen  of  0.46  carbon  steel  quenched  in  water  after  heating  15  minutes 
at  750°  C  (1380°  F),  just  above  the  Ai  transformation.  The  structure  consists  of  ferrite  and  martensite. 
X  500.  (6)  Specimen  similar  to  a,  quenched  in  water  after  heating  15  minutes  at  850°  C  (1515°  F),  just 
above  the  A2  transformation.  The  structure  consists  entirely  of  very  fine  martensite.  X  500.  (c)  Speci- 
men similar  to  a,  quenched  in  water  after  heating  15  minutes  at  1200°  C  (2190°  F).  A  very  coarsely  grained 
martensitic  structure  has  resulted,  in  which  intercrystalline  quenching  cracks  are  abundant.  X  100. 
Etching  medium,  2  per  cent  alcoholic  solution  of  nitric  acid 


Structure  and  Properties  of  Metals  71 

cylinder  (at  least  at  the  end  where  failure  occurred)  was  com- 
prised of  martensite  and  troostite,  that  the  tempering  operation 
for  this  particular  cylinder  was  omitted  through  some  oversight, 
and  that  the  subsequent  failure  of  the  material  resulted  from 
faulty  heat  treatment,  although  the  cylinder  successfully  with- 
stood the  required  hydraulic-pressure  test. 

Undesirable  properties  of  steel  are  sometimes  attributed  to 
faulty  heat  treatment  used  for  the  material,  a  microscopic  exami- 
nation of  which  shows  that  the  cause  is  an  entirely  different  one. 
A  section  of  a  forging  of  nickel  steel  intended  for  rifle  parts  was 


mm 


FIG.  44. — Microstructure  of  a  heat-treated  steel  container  for  compressed  gas  which  failed 

in  service.     X  250 

(a)  Structure  of  one  of  the  fragments  from  near  the  point  at  which  the  explosion  occurred;  the  material 
consists  of  martensite  and  troostite.  Etching  reagent,  i  per  cent  alcoholic  solution  of  hydrochloric  acid. 
(6)  Structure  of  a  specimen  of  steel  similar  to  a,  showing  the  microstructure  desired  and  which  should 
have  resulted  from  the  specified  treatment.  Etching  reagent,  2  per  cent  alcoholic  solution  of  nitric  acid. 
It  is  evident  that  the  explosion  of  the  cylinder  was  the  result  of  a  lack  of  tempering  of  the  hardened  material 

submitted  for  examination.  Difficulties  had  been  encountered 
in  drilling,  and  the  attempt  made  to  overcome  these  by  various 
annealings  of  the  material.  The  microscopic  examination  of  the 
annealed  piece  revealed  amartensitic  core,  while  the  outer  portions 
were  of  the  usual  ferrite-pearlite  structure  (Fig.  45).  Subsequent 
chemical  analysis  showed  that  the  nickel  content  of  the  central 
portion  was  very  much  higher  than  that  of  the  outer  parts,  enough 
so  as  to  render  the  material  martensitic  even  upon  slow  cooling. 
Evidently  heat  treatment  could  not  be  expected  to  improve  the 
machining  properties  of  such  material;  the  remedy  had  to  be 
sought  in  the  melting  practice  used  in  the  production  of  the  steel. 


72 


Circular  of  the  Bureau  of  Standards 


An  interesting  example  of  a  defect  which  may  result  in  copper 
which  has  been  improperly  annealed  is  shown  in  Fig.  46.  The 
metal  was  rendered  brittle  and  useless  by  numerous  fine  inter- 
crystalline  cracks  throughout  the  interior.  This  is  to  be  attrib- 


FIG.  45. — Structure  of  a  defective  nickel  steel  forging  caused  by  improper  melting  practice 

Note  the  dark  central  streak  in  a.  This  was  of  a  martensitic  structure,  r,  and  rendered  the  material  too 
hard  to  machine  properly.  This  defect  could  not  be  remedied  by  heat  treatment,  (a)  Macrostructure 
of  a  longitudinal  section  deeply  etched  with  concentrated  hydrochloric  acid,  showing  the  core  caused  by 
the  nondiffusion  of  nickel  in  the  molten  steel.  X  i.  (6)  Microstructure  of  specimen  a,  as  received  from 
the  mill,  outside  the  core.  X  100.  (c )  Microstructure  of  the  core  of  specimen  a,  as  received  from  the  mill. 
The  nickel  content  of  this  part  was  so  high  that  it  remained  martensitic  even  upon  slow  cooling.  X  500. 
Etching  reagent,  6  and  c,  2  per  cent  alcoholic  solution  of  nitric  acid 

uted  to  the  action  of  the  atmosphere  in  which  it  was  heated  rather 
than  to  the  temperature  used.  The  particles  of  cuprous  oxide, 
which  are  always  present  to  some  extent  in  remelted  copper,  are 
reduced  by  hydrogen  or  other  reducing  gases  which  readily  pene- 


Structure  and  Properties  of  Metals  73 

trate  the  heated  metal.  The  pressure  of  the  gaseous  products 
resulting  from  their  action  upon  the  oxide  is  sufficient  to  produce 
the  internal  cracks  throughout  the  hot  metal. 

2.  SUPPLEMENT  TO  CHEMICAL  ANALYSIS 

Several  instances  have  already  been  given  to  show  how  a  knowl- 
edge of  the  structure  of  an  alloy  may  be  a  very  valuable  aid  in 
sampling  the  material  for  chemical  analysis.  This  is  particularly 
true  for  segregated  alloys  (Figs  2  and  16)  and  for  those  in  which 
liquation  readily  occurs  (Fig.  41). 


FIG.  46. — Microstructure  of  copper  which  was  spoiled  in  annealing  as  a  result  of  the 
nature  of  the  surrounding  atmosphere.     X  100 

Note  the  fine  intercrystallinc  cracks  which  formed  in  the  interior  of  the  metal.    Such   material  is 
commonly  termed  "gassed"  copper.     Etching  reagent,  ammonium  hydroxide  and  hydrogen  peroxide 

The  microscopic  method  is  of  decided  value  in  supplementing 
the  chemical  study  of  the  various  metallic  coatings  used  for  pro- 
tective purposes,  particularly  on  iron  and  steels.  In  Fig.  47  the 
complex  structure  of  a  "brass"  coating  is  shown.  The  coating 
in  reality  consists  of  three  layers,  one  of  nickel  and  two  of  brass. 
It  is  evident  that  with  this  information  in  mind,  the  chemical 
determination  of  this  coating  can  be  carried  out  and  interpreted 
in  a  much  more  logical  way  than  without  it.  Fig.  47  b  shows  the 
duplex  structure  of  electrolytic  deposit  of  alternate  layers  of 
copper  and  nickel.  The  microscopic  method  may  be  used  also 
in  determining  the  thickness  and  distribution  of  the  coating 


74 


Circular  of  the  Bureau  of  Standards 


material;  although  laborious,  this  method  is  often  the  only  one 
available  for  such  determination.  Fig.  48  illustrates  an  application 
of  this.  r4  >i 

By  means  of  microscopic  studies  of  the  constituents  comprising 
the  coating  of  tinned  copper,  an  explanation  has  been  found  for  the 
corrosion  pitting  of  copper  roofing  which  had  been  treated  with  a 
"protective"  coating  of  tin,  Fig.  49."  One  of  the  constituents 


FIG.  47. — Microstructure  of  complex  metallic  coatings  produced  by  electrolytic  deposition 

(a)  Cross  section  of  a  "brass"  coating  used  on  a  steel  base;  the  coating  consists  of  three  layers,  the  inter- 
mediate one  being  of  nickel  (X  500);  (6)  cross  section  of  an  electrolytic  deposit  consisting  of  alternate  layers 
of  nickel  (light)  and  copper  (crystalline),  the  metal  being  deposited  in  the  direction  shown  by  the  arrow 
(X  100).  Etching  reagent,  ammonium  hydroxide  and  hydrogen  peroxide 

formed  by  the  interaction  of  tin  and  copper  is  electropositive  to 
copper,  the  two  being  in  contact  within  an  electrolyte,  and  thus 
it  stimulates  the  corrosive  attack  upon  the  latter  when  the  two 
are  exposed  simultaneously  to  corroding  influences.  Other  appli- 
cations of  the  use  of  the  metallographic  microscope  to  the  study 
of  metallic  coatings  have  been  discribed  in  a  previous  publica- 
tion.26 


K  P.  D.  Merica,  B.  S.  Tech.  Papers,  No.  90. 

*«  B.  S.  Circular  No.  80,  Protective  Metallic  Coatings  for  the  Rustproofing  of  Iron  and  SteeL 


Structure  and  Properties  of  Metals  75 

Because  of  the  relatively  higher  price  for  wrought  iron  as  com- 
pared with  mild  steel  there  is  at  times  a  tendency  to  " adulterate" 
this  product  with  additions  of  the  cheaper  metals.  While  the 
chemical  analysis  will  indicate  in  a  general  way  when  such  additions 
have  been  made,  the  metallographic  method  is  almost  indis- 
pensable for  quickly  revealing  the  extent  of  such  contaminations. 
Fig.  50  shows  the  appearance  of  a  specimen  of  commercial  wrought 
iron  which  has  been  suitably  prepared  to  show  the  results  pro- 
duced by  the  addition  of  low-carbon  steel  to  material  of  this  kind. 

Considerable  importance  is  attached  to  the  study  of  the  occur- 
rence of  gases  in  metals,  and  in  particular  those  gases  which  are 
given  off  by  steel  when  heated  in  vacuo.  Among  the  principal 


FIG.  48. — Variation  in  thickness  of  an  electrolytic  zinc  coating  suck  as  may  occur  on  flat 
sheets  after  plating,  determined  by  microscopic  measurements 

The  shaded  portion  represents  the  coating  drawn  to  the  scale  indicated  by  the  arrow,  the  length  of  which 
represents  0.094  mm.  The  cross  sections  of  the  plate  upon  which  the  coating  was  deposited  are  shown 
somewhat  less  than  natural  size,  the  longer  one  was  a  diagonal  section  of  a  4-inch  square  plate,  the  shorter 
one  was  a  parallel  section  K  inch  away 

gases  obtained  in  this  manner  is  carbon  monoxide.  A  study  of 
the  microstructure  of  specimens  of  steel  after  being  heated  in 
vacuo 27  shows  that  an  appreciable  decarburization  occurs  in 
such  material,  which  fact  throws  considerable  light  on  the  origin 
of  at  least  some  of  the  evolved  gases.  Fig.  51  shows  the  condi- 
tion existing  at  the  surface  of  a  specimen  of  low-carbon  steel 
(0.18  per  cent  carbon)  after  heating  in  vacuo  above  the  trans- 
formation temperature.  The  carbon  was  removed  for  a  consider- 
able depth  at  the  surface.  Evidently  a  chemical  reaction,  the 
reverse  of  that  by  which  carburization  of  steel  in  the  cementation 

17  H.  S.  Rawdon  and  H.  Scott,  Microstructure  of  Iron  and  Mild  Steel  at  High  Temperatures,  B.  S.  ScL 
Papers,  No.  356. 


76  Circular  of  the  Bureau  of  Standards 

or  casehardening  process  is  brought  about,  occurred  during  the 
heating.  In  the  usual  process  of  carburization,  carbon  penetrates 
the  metal  as  carbon  monoxide,  which  later  reacts  with  the  iron  to 
form  iron  carbide  (cementite)  and  carbon  dioxide,  which  diffuses 
outward  from  the  metal.  In  the  decarburization  of  steel  in 
vacuo  the  carbon  evidently  leaves  the  metal  as  a  gas;  this  would 
necessitate  the  presence  in  the  material  of  oxygen  in  some  form 
which,  by  a  reaction  with  the  carbide,  forms  the  gas  which  is  later 
removed  by  the  action  of  the  vacuum  pump.  Steels  made  under 


FIG.  49. — Appearance  of  the  exposed  surface  of  corroded  tinned  copper  cheating  used  for 
roofing  purposes.      X  i 

Note  the  corrosion  pits  which  occurred  in  service,  some  of  which  penetrated  through  the  sheet.     These 
were  the  result,  in  large  measure,  of  the  tin  coating  which  was  applied  to  the  copper 

widely  varying  conditions  differ  markedly  in  the  amount  of 
decarburization  which  occurs,  as  is  to  be  expected,  according  to  the 
amount  of  oxygen  retained  by  the  metal.  The  material  shown  in 
Fig.  5 1  was  a  laboratory  specimen  made  by  the  addition  of  carbon 
to  pure  iron,  no  further  additions  being  made  for  deoxidation  or 
other  purposes.  Hence  it  might  be  expected  that  such  material 
would  decarburize  much  more  rapidly  when  heated  in  vacuo  than 
a  steel  which  had  been  thoroughly  deoxidized  by  the  proper 
additions  to  the  molten  metal. 


Structure  and  Properties  of  Metals 


77 


The  chemical  determination  of  "slag"  in  steel  is,  at  its  best, 
rather  unsatisfactory.     Even  if  good  checks  are  obtained  in  dupli- 


•   :     V<-;J 

«-\x  ^m-t 

*•  ^     ''***   r.'^  <A     •  i  it\  .J3,  » 


>, 


,  * 


^  .'•  ^-^ 

-  <;;,-;,  ;-v 


FIG.  50.— Structure  of  -wrought  iron  to  which  additions  of  steel  have  been  made 
Note  the  white  streaks  a.  Such  an  appearance  after -suitable  etching  is  indicative  of  the  addition  of  steel 
to  wrought  iron,  (a)  Macrostructure  of  a  longitudinal  section  of  a  z-inch  round  of  wrought  iron,  deeply 
etched  with  hot  concentrated  hydrochloric  acid.  X  i .  The  white  streaks  represent  the  additjons  of  steel. 
(6)  Microstructure  of  the  metal  of  dark  portions  of  a.  This  has  the  characteristic  structure  of  wrought  iron. 
X  100.  (c)  Microstructure  of  metal  of  one  of  the  light  streaks  of  a.  The  metal  is  low-carbon  steel.  X  100 
Etching  reagent,  2  per  cent  alcoholic  solution  of  nitric  acid 

cate  determinations,  the  interpretation  of  the  results  is  a  matter  of 
considerable  difficulty.     That  this  must  be  so  is  evident  from  Fig. 


78  Circular  of  the  Bureau  of  Standards 

52,  which  shows  the  structure  of  a  slag  thread  from  wrought  iron. 
The  duplex  nature  of  this  simple  slag  is  plainly  shown,  and  it  is 
evident  in  the  case  of  the  complex  steels,  in  the  manufacture  of 
which  the  additions  to  the  metal  are  often  several  in  number  and 
varied  in  composition,  that  the  resulting  "slag"  must  be  cor- 
respondingly complex  in  its  nature. 

An  illustration  has  already  been  given  (Sec.  IV,  i;  Fig.  15) 
suggesting  how  it  is  possible  to  estimate  rather  closely  the  percent- 
age composition  of  certain  alloys  with  respect  to  certain  elements 
from  the  structural  appearance  of  the  material  alone,  if  the  alloy 
is  in  the  condition  of  stable  equilibrium.  This  method  is  of 


FIG.  51. — Microstructure  of  an   iron-carbon  alloy  which   uus  decarburized  by  heating  in 
•vacua .     X  100 

Note  the  absence  of  the  black  constituent  (the  carbon-bearin?  portions)  for  a  considerable  distance  below 
the  surface.  The  specimen  (0.18  per  cent  carbon)  was  heated  for  30  minutes  at  760°  C,  and  cooled  in  vacuo. 
The  cross  section  shows  the  distance  from  the  surface  to  which  the  carbon  has  been  removed;  etching 
reagent,  2  per  cent  alcoholic  solution  of  nitric  acid 

importance  from  a  practical  standpoint  in  the  estimation  of  oxy- 
gen in  copper,  a  determination  which,  if  carried  out  by  chemical 
means,  is  very  tedious  and  time  consuming.  Oxygen  in  copper 
exists  in  the  form  of  cuprous  oxide,  which  is  soluble  in  the  molten 
metal  and  upon  cooling  separates  as  the  copper-copper  oxide 
eutectic  (Fig.  57)  (3.45  per  cent  cuprous  oxide;  melting  point, 
1063°  C.)  in  a  manner  exactly  similar  to  certain  metals,  for  exam- 
ple, silver  and  copper.  From  an  estimation  of  the  relative  amount 
of  the  eutectic  in  cast  copper,  a  rather  close  value  of  the  percentage 
of  oxygen  present  can  be  computed  from  the  microstructure. 
Nickel  is  similar  to  copper  in  the  formation  of  an  oxide  eutectic. 


Structure  and  Properties  of  Metals  79 

The  microscope  is  often  an  indispensable  means  for  determin- 
ing the  r61e  played  by  certain  additions  made  to  alloys  in  the 
course  of  their  preparation.  This  is  true  particularly  for  steels. 
Thus  an  examination  of  the  structure  of  titanium-treated  steels 
shows  that  this  element,  titanium,  in  the  quantities  usually 


TRIO 

2&fPf 


FIG.  52. — Microstructure  of  wrought  iron  showing  the  complex  nature  of  the  slag 

(a)  Longitudinal  section  of  wrought  iron  showing  a  slag  streak  embedded  in  the  ferrite  crystals;  etching 
reagent,  2  per  cent  alcoholic  solution  of  nitric  acid.  X  100.  (6)  Slag  streak  similar  to  the  one  shown  in 
a,  unetched  X  500.  The  duplex  nature  of  the  slag  is  very  evident 

added  to  steel,  does  not  alloy  with  the  metal  in  the  sense  that 
many  added  elements  do.  Its  role  is  to  free  the  metal  of  unde- 
sirable substances  present  and  then  leave  the  metal  in  the 
slag,  carrying  the  detrimental  substance  combined  with  it.  It 
appears  to  be  especially  active  in  combining  with  nitrogen. 


go  Circular  of  the  Bureau  of  Standards 

Fig.  17  shows  one  of  the  characteristic  inclusions  found  in  steel 
which  has  been  treated  with  titanium.  Zirconium  appears  to 
act  in  a  manner  similar  to  that  of  titanium  when  added  to  steel 
in  small  amounts,  in  that  it  frees  the  metal  from  some  undesir- 
able substance  (not  necessarily  nitrogen)  and  then  escapes  from 
the  metal  in  some  combined  form  in  the  slag.  The  inclusions 
which  are  found  in  zirconium-treated  steel  are  of  a  very  char- 
acteristic shape  and  yellow  color  (Fig.  53).  On  the  other  hand, 
the  primary  role  of  other  added  elements  is  to  react  with  the  steel 
and  to  modify  the  properties,  either  of  the  ferrite  as  in  the  case 
of  nickel,  or  of  the  carbide  as  in  the  case  of  chromium. 


FIG.  53. — Microstructure  of  steel  to  which  additions  of  zirconium  have  been  made.      X  500 

The  small  light-colored  inclusions,  square  or  triangular  in  outline,  are  characteristic  of  steel  to  which 
zirconium  has  been  added.  They  are  of  a  striking  lemon-yellow  color.  Etching  reagent,  2  per  cent  alco- 
holic solution  of  nitric  acid 

The  form  in  which  certain  elements  occur  in  steel  and  their 
distribution  throughout  the  metal  are  often  of  much  more  im- 
portance, as  far  as  the  properties  of  the  metal  are  concerned, 
than  the  percentage  of  the  element  present.  This  has  already 
been  pointed  out  in  the  case  of  sulphur  (Sec.  IV,  i).  Phosphorus 
in  the  amount  usually  present  in  iron  and  steel  exists  in  solid 
solution  in  the  ferrite.  The  distribution  is  often  far  from  uni- 
form throughout  the  metal  (Fig.  5),  and  even  within  the  indi- 
vidual crystals  the  distribution  may  be  far  from  uniform,  as  is 
shown  in  Fig.  54. 


Structure  and  Properties  of  Metals 


81 


FIG.  54. — Microstructure  of  wrought  iron  of  high  phosphorus  content,  showing  the  lack  of 
uniform  distribution  of  this  element  even  within  the  individual  crystals.     X  100 

The  parallel  bands  within  the  crystals,  a,  are  indicative  of  very  brittle  iron,  (a)  Longitudinal  section 
of  wrought  iron,  high  in  phosphorus  (0.36  per  cent),  annealed  at  600°  C.  The  phosphorus  banding  within 
the  individual  grains  persisted  after  this  treatment:  etching  reagent,  10  per  cent  alcoholic  solution  of  nitric 
acid,  (i)  Longitudinal  section  of  high  phosphorus  wrought  iron,  etched  with  acidified  solution  of  cupric 
chloride  (Stead's  reagent) 

110580°— 22 6 


82  Circular  of  the  Bureau  of  Standards 

3.  CONTROL  OF  METALLURGICAL  OPERATIONS  AND  PRODUCTS 

A  knowledge  of  the  structure  of  an  alloy,  particularly  a  new 
one,  will  often  aid  materially  in  carrying  out  the  mechanical 
working  of  the  material  satisfactorily.  Fig.  55  shows  the  struc- 
ture of  a  boron  steel  (Fig.  55  a — carbon,  0.16  per  cent;  boron, 
0.49  per  cent;  nickel,  2.82  per  cent;  Fig.  55  b — carbon,  o.  16  per 
cent;  boron  0.39  per  cent).  When  the  attempt  was  made  to  roll 
these  steels  under  the  same  conditions  used  with  satisfactory 
results  for  steels  of  similar  composition  but  containing  no  boron, 
the  metal  crumbled  and  cracked  badly  in  the  rolls.  Indeed, 
some  ingots  were  so  brittle  that  they  broke  under  their  own 
weight  when  carried  from  the  furnace  to  the  rolls.  By  heating 
the  ingots  for  a  sufficiently  long  time  until  considerable  coales- 
cence of  the  eutectic  occurred,  and  by  reducing  the  temperature 
of  the  ingot  somewhat,  no  unusual  difficulties  on  working  the 
metal  were  encountered.  In  a  similar  way  oxide  films  and  similar 
inclusions  within  a  metal  will  often  prevent  such  materials  from 
being  worked.  In  such  cases  a  preliminary  treatment  of  the 
molten  metal  must  be  made,  usually  a  deoxidation  treatment, 
for  removing  these  undesirable  features.  Nickel  is  a  good  ex- 
ample; unless  the  metal  is  treated  with  magnesium  or  a  similarly 
acting  substance,  it  can  not  be  satisfactorily  rolled  or  worked  hot 
in  any  way. 

The  conditions  under  which  metals  are  cast  have  an  important 
bearing  upon  the  properties  of  the  resulting  castings.  The  ex- 
amination of  the  microstructure  often  reveals  features  due  to 
this  cause  which  render  the  metal  entirely  unfit  for  use,  although 
from  a  surface  inspection  the  metal  would  be  considered  suitable 
for  use.  Fig.  3  shows  a  section  of  a  large  steel  casting  for  a 
rudder  frame  which,  although  it  had  received  the  specified  heat 
treatment  and  had  been  passed  by  the  inspectors,  broke  during 
shipment,  before  it  was  ever  put  into  use.  The  metal,  which 
contained  numerous  inclusions  and  pores,  though  refined  as  to 
grain  size  by  suitable  heat  treatment,  still  retained  the  initial 
dendritic  or  ingot  structure  because  of  the  included  impurities, 
and  thus  also  the  accompanying  inferior  properties.  Tempera- 
ture of  casting  has  also  an  important  effect  on  the  properties  of 
cast  metal.  If  too  cold,  numerous  "cold  shuts,"  oxide  films,  and 
similar  undesirable  features  are  apt  to  occur  on  account  of  the 
sluggish  flow  of  the  metal.  Fig.  56  illustrates  such  a  condition 
in  cast  bronze.  On  the  other  hand,  if  too  hot,  a  very  coarsely 


Structure  and  Properties  of  Metals 


FIG.    55. — Microstructure  of  low  carbon  steels  to  which  additions   of  boron   have   been 
made.     X  500 

(a)  Section  of  an  ingot  of  boron  steel  which  broke  in  the  rolls  and  could  not  be  worked.  The  addition 
of  the  boron  caused  the  formation  of  the  eutectic  shown;  etching  reagent,  2  per  cent  alcoholic  solution  of 
nitric  acid.  (6)  Longitudinal  section  of  a  rolled  plate  of  boron  steel,  showing  the  coalescence  of  the 
eutectic  which  has  occurred.  Etching  reagent,  hot  alkaline  solution  of  sodium  picrate.  This  reagent 
colors  the  carbide  a  dark  brown  and  sometimes  nearly  black 


84  Circular  of  the  Bureau  of  Standards 

crystalline  state  results,  the  sand  of  the  mold  is  rapidly  eroded} 
and  particles  may  be  inclosed  within  the  metal,  gases  absorbed 
by  the  metal  during  the  overheating  may  be  released,  thus  giving 


FIG.  56. — Structure  of  inferior  zinc  bronze  castings  showing  defects  caused  by  improper 
conditions  of  casting 

Note  the  fine  cracks  within  the  individual  crystals,  a.  The  material  is  very  much  weakened  by  such 
a  network  as  in  b.  (a)  Macrostructure  of  cast  bronze  (approximate  composition;  copper,  88  per  cent; 
tin,  10  per  cent;  zinc,  2  per  cent),  showing  both  intercrystalline  and  intracrystalline  defects.  Etching 
reagent,  alcoholic  solution  of  ferric  chloride.  X  5.  (b)  Defective  cast  bronze  similar  to  a;  numerous  films, 
presumably  oxide,  and  discontinuities  exist  between  the  branches  of  the  dendritic  crystals;  etching 
reagent,  ammonium  hydroxide  and  hydrogen  peroxide.  X  100 

rise  to  inferior  castings,  and  undue  segregation  will  readily  occur 
in  large  castings  during  the  very  slow  cooling. 

The  microscope  is  often  valuable  in  determining  the  nature  of 
certain   metallurgical    products   masquerading   under   misleading 


Structure  and  Properties  of  Metals  85 

names.  Thus,  for  example,  the  nature  of  wrought  iron  can  be 
established  with  certainty  by  this  means,  "semisteel"  can  be 
shown  to  be  only  a  particular  grade  of  cast  iron,  having  none  of 
the  characteristic  properties  of  steel.  An  interesting  example 
which  may  be  cited  is  that  of  a  specially  prepared  copper,  adver- 
tised and  sold  as  a  deoxidizer  for  copper  and  other  alloys.  The 
examination  of  the  material  showed  that  the  deoxidizer  itself 
contained  very  appreciable  amounts  of  cuprous  oxide  (Fig.  57). 
The  "special  treatment"  given  the  material  to  render  it  a  deoxi- 
dizer was  not  sufficient  to  prevent  oxidation  of  the  metal  from  occur- 
ring during  the  process  of  treatment.  It  appears  plainly  evident, 


" 


FIG.  57. — Microslructure  of  a  proprietary  copper  alloy  recommended  by  the  "inventor" 
for  purposes  of  deoxidation  of -various  alloys.     X  5°° 

The  metal  as  received  from  the  makers  contained  numerous  inclusions  of  cuprous  oxide.     Etching 
reagent,  concentrated  ammonium  hydroxide. 

then,  that  such  a  substance  can  have  no  appreciable  deoxidizing 
effect  upon  an  alloy  to  which  it  might  be  added.  From  time  to  time 
the  Bureau  of  Standards  has  received  for  examination  numerous 
specimens  designated  as  "hardened  copper,"  considered,  according 
to  the  maker,  as  a  rediscovery  of  "the  lost  art  of  tempering  cop- 
per." A  microscopic  examination,  supplemented  at  times  by  a 
simple  chemical  analysis,  is  usually  sufficient  to  show  the  nature 
of  the  hardening  process.  A  favorite  method,  probably  unwit- 
tingly carried  out,  is  to  manipulate  the  melting  process  so  that 
the  metal  becomes  contaminated  with  oxide.  This  results  in  a 
much  harder  metal,  but  of  course  renders  it  unsuitable  for  the 
purposes  for  which  copper  is  used. 


86  Circular  of  the  Bureau  of  Standards 

Certain  metallurgical  processes  are  most  readily  and  surely  con- 
trolled by  means  of  examinations  of  the  structure  of  the  product 
at  various  stages  of  the  process.  The  manufacture  of  malleable 
iron  offers  a  good  example  of  this,28  as  does  also  its  converse,  the 
cementation  process.  The  structural  changes  which  occur  during 
such  processes  as  welding  2!)  and  oxyacetylene  cutting  of  metals 
affect  the  properties  of  the  materials  often  to  a  very  marked 
extent.  Fig.  58  shows  a  block  of  steel  which  has  been  cut  by  the 
oxyacetylene  flame.  The  metal  has  been  affected  to  a  very 
appreciable  depth.  An  examination  of  this  specimen  after  it 
had  been  annealed  showed  that  the  surface  change  in  the  metal, 
which  at  first  sight  may  be  mistaken  for  a  carburized  layer, 


FIG.  58. — Macrostructure  of  a  steel  block  showing  surface  changes  caused  by  cutting  the 
steel  by  means  of  the  oxyacetylene  fame.     X  I 

The  surface  was  appreciably  hardened  for  a  considerable  depth  by  the  chilling  action  of  the  cool  metal 
of  the  interior  upon  the  hot  metal  of  the  surface  after  the  flame  was  withdrawn.  This  sometimes  causes 
cracks  to  form  in  the  steel.  It  has  also  found  special  application  for  the  surface  hardening  of  steel.  Etch- 
ing reagent,  aqueous  solution  of  ammonium  persulphate 

was  the  result  of  the  "quenching"  action  of  the  cold  interior 
upon  the  hot  metal  at  the  surface  after  the  removal  of  the  flame. 
This  special  application  of  the  action  of  a  flame  upon  steel  has 
been  perfected  and  patented  for  the  surface  hardening  of  complex 
steel  shapes,  which  would  be  distorted  or  would  crack  if  hardened 
in  the  usual  manner. 

In  the  study  of  the  mechanical  working  of  metals  it  is  often 
necessary  to  follow  the  material  through  the  various  stages 
through  which  it  passes.  In  this  connection  the  chemical  un- 
homogeneity  of  metal,  as  has  been  indicated  previously,  serves 
a  useful  purpose;  this  is  true  in  particular  for  phosphorus  segre- 
gation, as  this  element  diffuses  extremely  slowly  so  that  the 
"flow  lines"  are  clearly  shown  by  its  presence  (Fig.  5).  Other 

M  Enrique  Tonceda,  "Research  work  on  malleable  iron,"  Mechanical  Engineering  Journal,  41,  p.  593; 
1919- 

"  H.  S.  Rawdon,  E.  C.  Groesbeck,  and  L.  Jordan.  Electric- Arc  Welding  of  Steel— Properties  o  5  the  Arc- 
Fused  Metals,  B.  S.  Tech.  Papers,  No.  179;  1920. 


Structure  and  Properties  of  Metals  87 

structural  features  often  answer  the  purpose  in  other  alloys; 
thus,  as  is  shown  in  Fig.  59,  certain  discontinuities  within  a 
wrought  round  bar  of  Monel  metal  were  clearly  shown  by  the 


FIG.  59. — Structure  of  monel  metal  showing  interior  defects  originating  before  the  metal 
was  rolled 

(a)  Monel  metal  rod  showing  an  internal  flaw.  Xi.  (ft)  Longitudinal  section  of  sound  monel  metal 
showing  "work  lines."  Xioo.  (c)  Section  through  the  defect  in  specimen  a.  The  distortion  of  the 
"work  lines"  in  the  neighborhood  of  the  defect  proves  that  the  defect  existed  in  the  metal  previous  to 
the  rolling  of  it.  X$o.  Etching  reagent,  6  and  r,  concentrated  nitric  acid 

characteristic  "work  lines  "  to  have  had  their  origin  while  the  metal 
was  in  a  plastic  state  and  not  to  have  been  produced  by  any 
treatment  given  the  material  subsequently  by  the  user. 


88  Circular  of  the  Bureau  of  Standards 

4.  CONSTRUCTION  OF  CONSTITUTIONAL  DIAGRAMS 

In  the  construction  of  the  constitutional  or  equilibrium  diagram 
of  any  alloy  system,  microscopic  examination  of  the  various 
preparations  forms  an  important  part  of  the  study.  Although 
the  data  obtained  by  the  thermal  study  form  the  basis  of  the 
work  by  furnishing  the  framework  of  the  diagram,  there  are  many 
features  which  must  be  checked  by  other  means  and  some  which 
can  be  determined  by  microscopic  study  only.  The  horizontal 
lines  of  a  constitutional  diagram  are,  in  general,  readily  estab- 


«7/r"  coppe 


FIG.  60. — Portion  of  the  constitutional  diagram  of  the  copper-aluminum  and  of  the 
magnesium-aluminum  alloy  system  illustrating  the  use  of  microscopic  examinations  in 
the  preparation  of  such  diagrams 

lished  by  the  method  of  thermal  analysis;  the  vertical  ones,  those 
which  mark  the  composition  boundaries  of  structural  "fields," 
depend  almost  entirely  upon  the  microscopic  study.  An  example 
of  this  use  of  the  microscope  is  illustrated  in  Fig.  60,  which  shows 
how  the  limit  of  solubility  of  certain  of  the  intermetallic  com- 
pounds which  occur  in  aluminum  alloys  was  determined  by  the 
method  of  prolonged  heating  of  specimens  at  certain  temperatures 
followed  by  microscopic  examination  of  the  heated  specimens. 
The  matter  of  solubility  of  these  compounds  is  of  very  considerable 
importance  in  connection  with  the  subject  of  the  hardening  of 
aluminum  alloys  upon  aging.30  The  microscopic  method  is  also 

30  P.  D.  Merica,  R.  G.  Wallenberg,  and  H  Scott,  The  Heat  Treatment  of  Duralumin,  B.  S.  Sci.  Papers, 
No.  347;  1919. 


Structure  and  Properties  of  Metals  89 

used  extensively  in  verifying  the  conclusions  tentatively  arrived 
at  as  a  result  of  the  data  obtained  by  thermal  analysis  or  other 
means. 

5.  FAILURE  OF  METALS  IN  SERVICE 

The  investigation  of  the  failure  of  metals  which  has  occurred 
during  service  requires  a  wide  and  comprehensive  knowledge  of 
the  properties  and  structure  of  metals,  both  sound  and  defective. 
The  microscope  has  proved  a  very  valuable  aid  in  such  studies 
and  the  structure  often  reveals  evidence  bearing  upon  the  cause 
of  failure  which  the  ordinary  methods  of  testing  fail  to  detect. 
To  discuss  the  relation  of  the  microstructure  to  the  failure  of 
metals  is  impossible  here;  brief  references  only  to  a  few  specific 
cases  will  be  used  as  examples. 

Fig.  6 1  shows  the  appearance  of  a  fracture  which  occurred  in 
service  in  a  lo-inch  steel  shafting.  The  fracture  had  the  charac- 
teristic appearance  of  a  "detail"  or  fatigue  break  which  started 
in  the  angle  of  a  keyway.  The  examination  of  the  structure 
showed  that  it  was  almost  a  perfect  one  for  such  a  failure ;  that  is, 
for  the  particular  composition  of  steel  used  (Fig.  6 1  6) .  Evidently 
the  material  had  received  no  annealing  treatment  whatever  for 
grain  refinement  after  the  forging  of  the  shaft  was  completed. 
The  large  crystals  with  their  prominent  Widmanstattian  structure 
are  almost  perfect,  so  far  as  failure  to  withstand  repeated  or 
vibratory  stresses  is  concerned. 

Fig.  62  shows  the  appearance  of  the  fractured  face  of  a  Muntz 
metal  bolthead  which  dropped  off  in  service  "of  its  own  accord." 
Examination  of  the  microstructure  showed  that  the  failure 
was  the  result  of  selective  corrosion  of  the  ^-constituent,  local- 
ized at  the  apex  of  the  angle  between  the  shank  and  head 
and  accelerated  by  the  service  stress  carried  by  the  bolt.  It  can 
readily  be  shown  31  that  corrosion  of  such  alloys  at  the  apex  of 
a  narrow  groove  (Fig.  63)  is  much  more  intense  than  elsewhere 
in  the  same  material  when  the  specimen  is  subjected  to  a  tensile 
stress  while  surrounded  by  the  corroding  agent— for  example,  sea 
water.  This  behavior  is  to  be  attributed  to  the  fact  that  the 
stress  carried  was  not  distributed  equally  in  all  cross  sections,  but 
was  much  higher  at  the  bottom  of  the  V  groove  than  elsewhere. 
Under  the  combined  action  of  corrosion,  of  the  sea  water  and 
the  service  tensile  stress  carried  by  the  bolt,  the  portion  of  the 


«  H.  S.  Rawdon,  Typical  Cases  o!  the  Deterioration  of  Muntz  Metal  by  Selective  Corrosion,  B.  S.  Tech. 
Papers,  No.  103.  P.  D.  Merica,  Failure  of  Brass:  2—  Effect  of  Corrosion  on  the  Ductility  and  Strength  of 
Brass,  B.  S.  Tech.  Papers,  No.  83. 


Circular  of  the  Bureau  of  Standards 


FIG.  61. — Structure  of  a  steel  shafting  which  failed  in  service  by  fatigue 

Note  the  excessively  large  grain  size,  6.  This  was  responsible  for  the  service  failure  of  the  shafting. 
(a)  Face  of  the  fracture,  which  originated  in  a  keyway.  X  %.  (6)  Microstructure  oi  the  metal  oi  the 
interior.  X  50.  Evidently  the  forging  received  no  treatment  for  grain  refinement.  Etching  reagent,  2  per 
cent  alcoholic  solution  of  nitric  acid 


Structure  and  Properties  of  Metals 


IHHHHHIHHHH 


.- 


FlG.  6s. — Structure  of  a  brass  (Muntz  metal)  bolt  which  failed  in  service,  evidently  by 
being  corroded  while  stressed  in  tension 

(a)  Fractured  face  of  the  bolt  head  which  dropped  off  of  its  own  accord.  X  i.  (6)  Longitudinal  section 
of  the  bolt  just  before  the  head  was  detached.  (<:)  Microstructure  of  the  metal  at  the  fracture  in  the  por- 
tion i— i'  of  a  and  6.  The  (3  constituent  has  been  dezincified  by  the  corrosive  action  of  the  sea  water  to  a 
considerable  depth  (black  in  micrograph)  by  corrosion.  X  250.  (d)  Microstructure  of  the  metal  at  the 
fracture  in  the  central  portion,  x—x  in  a  and  6.  No  corrosion  has  occurred  here;  tensile  stress  alone  caused 
the  fracture  of  this  part.  X  250.  Etching  reagent,  ammonium  hydroxide 


92  Circular  of  the  Bureau  of  Standards 

fracture  x-x'  was  gradually  produced;  this  has  the  appearance  of 
a  "detail"  or  fatigue  break.  When  the  cross-sectional  area 
became  small  enough  so  as  to  break  under  the  applied  loading 
the  central  portion  broke  as  a  simple  tensional  break.  The 
microstructure  of  the  two  portions  confirms  this;  in  the  portion 
x-x'  the  /?  constituent  at  the  extreme  edge  of  the  fracture  showed 
evidence  of  deterioration  by  dezincification,  while  in  the  central 
portion  the  alloy  was  sound  and  unchanged  up  to  the  extreme 
edge  of  the  fracture. 


FIG.  63. — Microstructure  of  corroded  Muntz  metal,   illustrating  the  effect  of  stress  in 
localizing  the  corrosive  action  of  this  alloy.     X  250 

A  tensile  specimen,  %  inch  diameter,  encircled  by  a  sharp  narrow  V-groove,  was  immersed  in  a  5  per  cent 
solution  of  sodium  chloride  while  under  stress.  The  micrograph  shows  a  section  of  the  metal  at  the  apex 
of  the  groove.  Dezincification  of  the  /3  constituent  appears  to  have  been  accelerated  by  the  relatively 
higher  stress  at  the  bottom  of  the  groove.  Etching  reagent,  ammonium  hydroxide  containing  ammonium 
persulphate 

Fig.  1 6  illustrates  a  failure  which  occurred  in  a  railroad  rail 
on  account  of  the  poor  quality  of  the  steel  used.  The  rail  showed 
evidence  of  a  high  degree  of  segregation,  the  portion  adjacent  to 
the  split  which  occurred  being  of  approximately  1.2  per  cent 
carbon  content,  while  the  outer  was  much  nearer  the  normal  com- 
position. Under  the  high  pressure  of  the  wheel  loads  the  hard 
central  streak  was  gradually  shattered  and  the  break  extended 
until  the  split  through  the  entire  head  of  the  rail  resulted.  There 
can  be  no  doubt  that  the  highly  segregated  nature  of  the  metal  was 
responsible  for  the  failure  in  this  case.  It  is  not  to  be  inferred, 


Structure  and  Properties  of  Metals  93 

however,  that  as  a  rule  all  segregated  rails  fail  in  service.  The 
degree  of  segregation  and  the  intensity  and  character  of  the 
service  stresses  decide. 

6.  SERVICE  DETERIORATION  OF  ALLOYS 

It  sometimes  happens  that  a  metal  or  an  alloy  used  for  some 
specific  purpose  deteriorates,  usually  in  a  chemical  way,  under 
the  peculiar  conditions  to  which  it  is  exposed,  although  for  other 
purposes  and  for  other  conditions  the  material  is  suitable  in 
every  respect.  An  illustration  of  such  deterioration  is  shown  in 
Fig.  64,  which  represents  an  alloy  essentially  of  aluminum  and 
zinc  (zinc,  85  per  cent)  used  as  a  cover  for  a  fuse  box.  The  ma- 
terial was  used  in  a  tropical  climate,  and  hence  was  exposed  to 
conditions  of  high  humidity  and  temperature,  and  the  cover, 
originally  flat,  warped  severely  and  bent  out  of  shape.  The 
surface  showed  characteristic  "alligator"  cracks.  Fig.  64  c  and  d 
shows  the  appearance  of  two  test  specimens  of  a  somewhat  similar 
alloy  intended  for  die  castings  after  exposure  to  dry  and  to  moist 
heat  (100°  C).  Heat,  in  the  absence  of  moisture,  had  no  appre- 
ciable effect  upon  the  material;  the  specimen  retained  its  initial 
dimensions  and  shape.  However,  the  sample  exposed  to  the 
combined  action  of  heat  and  moisture  rapidly  deteriorated  by 
permanent  expansion  and  distortion  of  the  piece.  An  appre- 
ciable increase  of  hardness  also  accompanied  this  change.  Micro- 
scopic examination  showed  that  the  eutectic  was  the  portion 
attacked;  presumably  oxidation  took  place  with  accompanying 
increase  of  volume,  the  presence  of  moisture  being  necessary  for 
this  change  to  occur. 

A  somewhat  similar  change  sometimes  occurs  in  the  filling 
used  for  fusible  boiler  plugs.  The  filling  prescribed  for  such 
plugs  is  very  high-grade  tin.  The  presence  of  impurities  in 
small  amounts,  particularly  zinc,  has  been  found  32  to  stimulate 
the  oxidation  of  the  tin  filling  so  that  in  time  the  plug  filling 
becomes  a  hard  infusible  mass  of  oxide  as  shown  in  Fig.  65. 
In  this  case,  as  the  above,  the  eutectic  is  the  constituent  which 
is  most  rapidly  attacked  and  oxidized  under  the  combined  action 
of  heat  and  moisture.  A  very  considerable  increase  in  volume 
accompanies  the  change  in  the  tin  filling  of  such  plugs. 

A  somewhat  different  type  of  deterioration  is  illustrated  in 
Fig.  66.  The  terminals  in  spark  plugs  are  very  often  made  of 

w  G.  K.  Burgess  and  P.  D.  Merica,  An  Investigation  of  Fusible  Tin  Boiler  Plugs,  B.  S.  Tech.  Papers, 
No.  53. 


94 


Circular  of  the  Bureau  of  Standards 


FIG.  64. — Appearance  of  certain  aluminum  alloys  of  high  zinc  content  after  deterioration 


(a)  Fusebox  cover, approximate  composition:  Aluminum,  is  per  cent;  lead,  0.4 per  cent;  zinc,  remainder; 
which  was  used  in  a  tropical  climate  and  warped  in  service.  X  }.».  (6)  Surface  of  specimen,  showing  "alli- 
gator cracks."  X  i.  (c)  Cylinder  of  an  alloy,  composition  copper,  1.4  per  cent:  aluminum,  15.2  per  cent, 
zinc,  83.4  per  cent,  exposed  to  "dry  heat"  (100°  C)  for  6  days.  The  dimensions  of  the  specimen  remained 
unchanged.  X  1%.  (d)  Specimen  similar  to  c  exposed  to  "moist  heat"  (100°  C)  for  6  days.  The  specimen 
expanded  and  cracked.  X  ilA.  (e)  Cross  section  of  specimen  similar  to  d,  showing  th>  "expansion 
cracks"  which  formed  in  the  metal  near  the  surface  by  the  action  of  heat  and  moisture;  most  of  these  occur 
in  the  eutectic;  the  specimen  was  unetched.  X  ice 


Structure  and  Properties  of  Metals 


95 


nickel  wire;  these  nickel  terminals  show  more  or  less  deteriora- 
tion in  all  spark  plugs  on  account  of  the  high  temperature  and 


FIG.  65. — Structure  of  safety  boiler  plugs  "which  deteriorated  in  service 

Note  the  very  considerable  increase  in  volume  of  the  filling  of  the  plug  which  resulted  from  the  deteriora- 
tion, (a)  Longitudinal  section  of  a  "safety"  plug,  to  the  deterioration  of  which  was  attributed  the 
explosion  of  a  marine  boiler  with  considerable  loss  of  life.  The  filling  of  tin  was  changed  almost  com- 
pletely by  service  conditions  into  a  hard  refractory  mass  of  tin  oxide;  a  few  globules  of  tin  may  still  be 
seen  in  the  interior.  X  i.  (6)  Same  specimen  as  a;  the  portion  containing  the  filling  has  been  polished 
so  as  to  show  the  relative  amounts  of  oxide  and  tin.  The  white  areas  are  globules  of  tin;  the  remainder 
of  the  filling  consisted  of  stannic  oxide.  X  2.  (c)  Longitudinal  section  of  a  plug  used  for  four  months, 
the  tin  filling  of  which  has  begun  to  deteriorate.  The  oxidation  of  the  tin  has  occurred  by  the  formation 
of  a  tree-like  network  permeating  the  tin.  X  i 

perhaps  other  conditions  accompanying  their  use.     This  deterio- 
ration consists  of  an  intercrystalline  network  which  may  in  time 


96 


Circular  of  the  Bureau  of  Standards 


FIG.  66. — The  appearance  of  deteriorated  nickel  -wire  and  tension  specimens  of  the 
tested  at  a  high  temperature 


Note  the  breaks  in  the  wires  of  the  spark  plugs,  a.     These  were  the  result  of  an  intercrystalline  attack  of 
d.    (a)  End  view  of  four  spark  plugs  the  nickel  electrodes  of  which  have 
deteriorated  in  service.    The  wires  have  been  severed  transversely.    X  i.     The  wires  are  anchored  firmly 


the  nickel  wire,  as  shown  in  b  and  d.     (a)  End  vie\ 


at  both  ends  so  that  considerable  stress  may  be  set  up  by  differential  expansion  when  the  plug  becomes 
heated.  (6)  longitudinal  section  through  one  of  the  nickel  wires  of  a,  showing  the  network  of  intercrys- 
talline fissures  which  formed  in  the  metal  during  service,  by  which  the  severing  of  the  electrode  wires 
occurred.  X  100.  (c)  Nickel  wires  broken  by  stressing  in  tension  at  approximately  900°  C.  The  metal 
behaves  as  a  brittle  material  under  these  conditions.  X  5-  (d)  Longitudinal  section  of  one  of  the  wires 
of  c;  intercrystalline  cracks  and  fissures  identical  in  appearance  with  those  of  b  have  formed  in  the  metal. 
Xioo.  (e)  End  of  a  nickel  wire  broken  similarly  to  those  of  c.  The  fracture  was  entirely  intercrystalline 
in  its  nature.  X  25.  Etching  reagent,  b,  d,  and  c ,  concentrated  nitric  acid. 


Structure  and  Properties  of  Metals  97 

develop  into  visible  intercrystalline  fissures  or  cracks.  A  study  33 
was  made  of  various  types  of  spark  plugs,  and  it  was  found  that 
in  plugs  of  certain  design  the  nickel  terminals  were  subject  to 
more  or  less  tensional  stress  while  hot,  and  in  such  cases  the  de- 
terioration was  very  pronounced — very  deeply  penetrating  cracks 
formed  which  soon  severed  the  entire  wire.  It  was  shown  by 
the  microscopic  examination  of  similar  wires  which  had  been 
subjected  to  the  combined  action  of  tensional  stress  and  heat 
that  the  material  deteriorated  in  a  manner  identical  in  appear- 
ance with  that  of  the  terminals  of  the  spark  plugs  in  service 
(Fig.  66  c,  d,  and  e) . 

7.  MISCELLANEOUS 

A  knowledge  of  the  microstructure  of  metals  and  alloys  is  also 
useful  in  a  great  number  of  miscellaneous  ways,  of  which  the 
following  are  mentioned  as  interesting  and  typical  examples. 

(a)  ELECTROLYTICALLY  DEPOSITED  METALS. — Fig.  67  shows  the 
structure  of  copper  which  has  been  deposited  electrolytically 
under  different  conditions  of  current  density,  and  also  the  same 
materials  after  annealing.  Copper  deposited  slowly  forms  rela- 
tively large  regular  crystals  which  change  little,  if  any,  upon 
subsequent  annealing.  On  the  other  hand,  copper  deposited  at 
higher  current  densities  forms  smaller  and  more  irregularly  shaped 
crystals,  many  of  which  are  twinned.  Upon  annealing,  such 
deposits  behave  exactly  like  copper  which  has  been  previously 
distorted  by  cold  working.34  The  metal  recrystallizes,  grain 
growth  follows,  and  the  structure  presents  an  entirely  different 
appearance  from  that  of  the  metal  as  deposited.  It  has  been 
previously  shown  (Sec.  IV,  2,  b)  that  a  change  in  the  energy 
content  of  a  metal  by  means  of  permanent  distortion  by  cold 
working  or  similar  means  is  a  necessary  condition  for  recrystal- 
lization  of  that  metal  upon  annealing.  It  would  appear,  then, 
that  metals  deposited  electrolytically  may  differ  very  materially 
in  this  same  respect,  according  to  the  conditions  under  which 
they  are  laid  down. 

(6)  BRINELL  HARDNESS  TESTS. — An  interesting  practical  appli- 
cation of  a  knowledge  of  the  microstructure  of  steel  is  illustrated 
in  Fig.  68.  This  shows  how  advantage  may  be  taken  of  the 
etching  properties  of  the  material  of  the  steel  ball  used  in  the 

»  H.  S.  Rawdon  and  A.  I.  Krynitzky,  A  Study  of  the  Deterioration  of  Nickel  Spark-Plug  Electrodes 
in  Service,  B.  S.  Tech.  Papers,  No.  143;  1920. 

M  H.  S.  Rawdon,  "Note  on  the  occurrence  and  significance  of  twinned  crystals  in  electrolytic  copper," 
Am.  Inst.  of  Metals,  10,  p.  198;  1916. 
110580°— 22 7 


Circular  of  the  Bureau  of  Standards 


FIG.  67. — Structure  of  electrolytic  copper  before  and  after  annealing 

(a)  Cross  section  of  a  sheet  of  electrolytic  copper  as  deposited;  current  density,  0.41  amperes  per  square 
foot;  temperature  of  solution,  38°  C.  The  initial  layer  adjacent  to  the  cathode  face  (right  side)  is  finely 
crystalline.  X  210.  (6)  Material  similar  too,  annealed  2  hours  at  approximately  600°  C.  The  finely  crys- 
talline layer  adjacent  to  the  cathode  face  has  recrystallized  and  shows  twinned  layers;  the  remainder,  or 
coarsely  crystalline  portion,  has  remained  unchanged.  X  250.  (c)  Cross  section  of  a  sheet  of  electrolytic 
copper  as  deposited;  current  density,  0.73  amperes  per  square  foot;  temperature  of  solution.  25°  C.  Small, 
irregularly  shaped  crystals,  many  of  which  are  twinned,  are  characteristic  of  this  type  of  deposit.  X  210. 
(</)  Material  similar  to  c  after  annealing  as  in  6.  X  210.  The  metal  has  entirely  recrystallized  and  grain 
growth  occurred  in  a  manner  identical  with  that  of  copper  which  has  been  annealed  after  being  cold- 
worked.  Etching  reagent,  ammonium  hydroxide  (i — i)  followed  by  hydrogen  peroxide.  In  each  case 
the  metal  was  deposited  in  the  direction  shown  by  the  arrow 


Structure  and  Properties  of  Metals 


99 


FIG.  68. — Appearance  of  the  etched  surface  of  a  ball  used  in  Brinell  hardness  determina- 
tions and  the  indentations  produced 

(a)  Etched  surface  of  hardened  steel  ball  (chromium  steel)  used  in  Brinell  hardness  determination; 
etching  reagent,  i  per  cent  alcoholic  nitric  acid.  The  white  particles  are  the  globules  of  carbide  and  are 
unattacked  by  moderate  etching.  X  500.  (6)  Brinell  indentations  produced  on  a  polished  specimen  of 
file  steel  (carbon  approximately  1.4  per  cent),  with  a  load  of  1000  kg  applied  for  30  seconds  on  a  lomm 
ball;  i  and  2  were  obtained  with  an  etched  ball,  3  with  a  polished  ball.  The  irregular  spot  at  3  is  an  ink 
mark  for  identification.  X  3.  (c)  Indentation  number  i  of  b.  X  50.  The  matt  appearance  of  the  inden- 
tations, i  and  2,  of  b  is  the  result  of  the  great  number  of  tiny  pits  produced  by  the  carbide  particles, 
which  cover  the  face  of  the  indentation.  The  indentation  shor  <>  two  distinct  concentric  zones,  appar- 
ently caused  by  a  difference  n  the  pressure  transmitted  to  the  steel  plate  being  tested 


ioo  Circular  of  the  Bureau  of  Standards 

Brinell  hardness  test  in  order  to  obtain  a  higher  degree  of  accuracy 
in  the  results  when  testing  hard  materials.  The  impression 
of  the  ball  upon  the  surface  of  a  hard  metal,  particularly  if  the 
latter  is  polished,  is  very  indefinite,  and  the  measuring  of  the 
diameter  of  the  impression  is  a  matter  of  considerable  uncertainty. 
The  ball  used  for  the  purpose  consists  practically  always  of  a 
chromium  steel  of  such  a  composition  that  in  the  hardened  state 
it  contains  free  carbide  in  the  form  of  numerous  rounded  particles. 
By  etching  the  ball  slightly  before  use  (2  minutes'  immersion  in  i 
per  cent  alcoholic  nitric  acid)  the  "flowed  metal"  on  the  polished 
surface  is  removed,  and  the  hard  carbide  particles  are  revealed. 
An  impression  made  with  such  an  etched  ball  upon  a  polished 


1  »*   *r  •&,'      ,-.  «*      *  • 


' '  ;•**•-'        *-» 

<  *    '/  .*'  .  -    *  ^  , 


FIG.  69. — Microstructure  of  leaded  brass.     X  ioo 

Each  black  spot  represents  a  globule  of  lead  added  for  the  purpose  of  improving  the  machining  prop- 
erties of  the  alloy;  the  specimen  was  unetched 

surface  of  a  hardened  steel  is  very  much  more  conspicuous  than  one 
obtained  with  a  polished  one  (Fig.  68  b) .  When  the  high  pressure 
is  applied,  the  carbide  particles  produce  a  multitude  of  tiny  pits 
over  the  surface  of  the  indentation  of  the  ball,  thus  giving  a  "  mat 
finish"  to  the  indentation  which  aids  very  materially  in  defining 
its  limits. 

(c)  WORKABILITY  OF  METALS. — The  successful  behavior  of 
metals  in  automatic  devices  for  turning  out  finished  articles,  for 
example,  the  manufacturing  of  screws  in  an  automatic  lathe, 
depends  largely  upon  the  ability  of  the  material  to  produce  turn- 
ings which  are  essentially  brittle  and  break  easily  so  that  no  clog- 
ging results.  Such  a  material  is  readily  obtained  in  the  case  of 
steel  by  the  addition  of  sulphur  to  the  metal  and  in  brass  by  the 
addition  of  lead.  Both  of  these  additions,  lead  in  brass  and  sul- 
phide in  the  steel,  exist  as  small  isolated  particles  which  break  up 


Structure  and  Properties  of  Metals  101 

the  continuity  of  the  metal  so  that  long  twisted  turnings  do  not 
result.  The  appearance  of  the  structure  of  metal  used  for  such 
purposes  is  shown  in  Figs.  18  and  69. 

The  structure  of  a  malleable-iron  casting,  in  the  automatic 
machining  of  which  considerable  difficulty  was  experienced,  is 
shown  in  Fig.  70.  The  malleableizing  process  was  carried  to  such 
an  extent  that  a  relatively  thick  surface  layer  of  ferrite  was  formed 
on  the  casting.  This  constituent  (pure  iron)  is  soft  and  has  very 
inferior  machining  properties. 

(d)  SERVICE  TEMPERATURE  OF  METALS.  —  A  record  of  the  tem- 
perature attained  by  some  metals  during  service,  for  example,  in 
bearings,  is  sometimes  recorded  in  the  metal  itself.     Fig.  71  shows 
the   appearance   of   a   bearing 
bronze    which    became    over- 
heated in  service.     The  structure 
of  the  metal  at  the  heated  sur- 
face was  found   to   be  identical 
with  that  of   the  same  bronze 
quenched    from   a  temperature 
slightly  above  500°  C.     At  this 

temperature  (approximately  500°  FlG-  1^-—  ^^restructure  of  a  malkable- 
^w  ,  ,  ,  •  j  ,  £  •  iron  casting  -with  which  difficulties  -were 

C)  the  eutectoid  transformation  exf>eriencjin  the  autojtic  machining 
occurs  in  bronze  containing  ap-  to  size,  x  / 

IO  per  Cent    Or    more  The  metal  at  the  surface  (light-colored  layer) 


Of    tin         It     is     evident     from     the        ^as  entirely  decarburized  in  the  "maUeablizing" 

process  and  a  thick  layer  of  ferrite  resulted.     This 

appearance  Of  the  Overheated  constituent  is  soft  and  "  gummy,  '  '  and  machines 
i  '  i  j  •  ,  1  with  difficulty.  The  specimen  is  unetched 

bearing  bronze  as  compared  with 

the  initial  structure  and  the  same  after  quenching  from  a  tem- 
perature above  that  of  the  eutectoid  transformation  that  the 
surface  metal  was  heated  to  a  temperature  at  least  above  that 
of  the  eutectoid  transformation  and  then  suddenly  chilled  by  the 
mass  of  cooler  metal  backing  it.  Similar  cases  are  encountered 
in  steel  and  other  alloys  which  undergo  a  structural  transforma- 
tion upon  heating. 

(e)  STRESS  DISTRIBUTION  IN  MECHANICAL  TEST  SPECIMENS.  —  As 
previously  stated  (Sec.  IV,  2,  6),  recrystallization  of  metals  upon 
annealing  necessitates  a  previous  distortion  of  the  crystals, 
usually  by  cold  working.  An  interesting  application  of  this  fact 
to  the  study  of  the  distribution  of  stress  in  specimens  used  for  the 
various  mechanical  tests  has  been  suggested  by  Chappel.35  The 
specimen,  which  should  be  annealed  first  to  remove  any  traces  of 

55  C.  Chappel,  "  Recrystallization  of  deformed  iron,  "  J.  Iron  and  Steel  Inst.  ,  89,  p.  460;  1914. 


IO2  Circular  of  the  Bureau  of  Standards 

previous  crystalline  distortion,  is  tested  and  then  annealed  at  a 
relatively  low  temperature,  which  in  the  case  of  steel  must  be 
below  the  transformation  temperature.  Recrystallization  and 


FIG.  71. — Micro  structure  of  a  bronze  used  for  a  bearing  ichich  became  overheated  in  service. 

X  500 

Note  the  light  constituent  which  appears  to  be  uniform  in  its  structure,  a.  This  appearance  indicates 
that  a  temperature  of  at  least  550°  C  was  reached  in  the  bearing,  (a)  Structure  of  the  alloy  adjacent  to  the 
heated  surface  of  the  bearing,  (b)  Structure  of  a  specimen  of  the  same  alloy  taken  somewhat  below  the 
bearing  surface;  the  alloy  has  its  normal  structure  and  consists  of  a  eutectoid  in  a  softer  matrix,  (c )  Speci- 
men similar  to  6,  quenched  in  water  after  heating  to  approximately  550°  C.  The  structure  is  identical 
with  that  of  the  alloy  at  the  bearing  surface.  Etching  reagent,  concentrated  ammonium  hydroxide 

grain  growth  will  occur  throughout  the  metal,  the  size  of  grain 
acquired  differing,  however,  in  different  portions  according  to  the 
stress  to  which  the  material  was  previously  subjected.  The 


Structure  and  Properties  of  Metals  103 

structure  of  the  annealed  specimen  reveals  then  in  an  interesting 
qualitative  way  the  distribution  of  the  stress  to  which  the  specimen 
was  subjected.  The  method  gives  most  interesting  results  in  the 
case  of  those  tests  in  which  a  pronounced  difference  in  the  stress 
distribution  occurs,  such  as  the  impact  or  shock  tests. 

VII.   INFORMATION  REGARDING  TESTS 

1.  REPORTS 

In  general  the  Bureau  will  not  in  a  formal  report  express  an 
opinion  as  to  the  suitability  of  any  metal  or  alloy  for  any  specific 
purpose,  this  restriction  to  apply  to  proprietary  alloys  in  particular. 
The  results  of  the  examination  by  which  the  properties  and  the 
structure  of  the  material  are  determined  will  be  given.  Photo- 
micrographs will  be  accompanied  by  statements  as  to  the  various 
constituents  which  are  shown  and  the  conditions  under  which 
the  examination  was  made.  In  describing  the  microstructure  of 
iron  and  steel  the  Bureau  will  conform,  in  general,  to  the  Nomen- 
clature of  the  Microscopic  Substances  and  Structures  of  Iron  and 
Steel,  recommended  by  the  sixth  congress  of  the  International 
Society  for  Testing  Materials. 

2.  TESTS 

For  a  proper  and  thorough  understanding  of  any  alloy  and  its 
properties  a  series  of  examinations  is  usually  necessary — chemical, 
mechanical,  thermal,  and  microscopic,  as  well  as  others  for  special 
purposes.  In  general,  chemical  analyses  will  be  made  by  the 
Bureau  for  individuals,  only  in  cases  of  dispute  or  when  some 
question  of  very  considerable  scientific  or  technical  importance 
is  involved. 

Most  of  the  tests  which  the  Bureau  is  called  upon  to  make  in 
connection  with  the  physical  metallurgy  of  metals  are,  with  few 
exceptions,  of  a  special  nature,  each  one  involving  considerable 
investigation.  The  Bureau  is  equipped  for  studying  in  detail 
the  methods  of  preparation  of  alloys  and  metals,  the  shaping  of 
such  metals  by  suitable  mechanical  working,  and  the  properties 
of  the  resulting  products,  together  with  such  related  subjects  as 
have  a  direct  bearing  upon  these  different  processes.  Such 
phases  of  the  preparation  of  metals,  as  ore  dressing,  smelting 
processes,  and  the  like,  are  not  included.  The  Bureau  of 
Standards'  examination  begins  with  the  metal  as  such.  The 
various  metallurgical  examinations  which  the  Bureau  is  equipped 
for  carrying  out  are  listed  below. 


104  Circular  of  the  Bureau  of  Standards 

(a)  MICROSCOPY  AND  STRUCTURE  OF  METALS. — Identification  of 
metallographic  constituents,  unknown  alloys,  previous  heat 
treatment  of  alloys,  mechanical  treatment  of  metals;  examination 
for  evidence  bearing  on  the  causes  of  service  failure  of  metals; 
study  of  methods  suitable  for  metal  microscopy,  including  etching 
reagents,  preparation  of  the  polished  surface,  etc.;  corrosion  and 
its  prevention  as  related  to  structure;  metallographic  apparatus. 

(6)  THERMAL  ANALYSIS  AND  HEAT  TREATMENT. — Location  by 
thermal  analysis  of  critical  transformation  temperatures  as  a 
check  upon  routine  methods  of  heat  treatment  and  for  establish- 
ing a  suitable  heat  treatment  for  new  alloys;  heat  treatment  of 
materials  submitted;  study  of  cementation  and  similar  processes, 
together  with  the  heat  treatment  necessary;  auxiliary  problems, 
furnace  control  for  heat  treatment,  efficiency  of  quenching 
mediums,  etc. 

(c)  WORKING  OK  METALS,  AND  RELATED  PROPERTIES,  SPECI- 
FICATIONS.— Mechanical  working  of  metals  submitted,  including 
forging,    rolling,    drawing,    welding,    etc.;   determination   of   the 
mechanical  uniformity  (initial  internal  stresses)  of  metals  after 
working  or  other  severe  treatments;  determination  of  the  char- 
acteristics and  behavior  of  bearing  metals,  safety  boiler  plugs, 
etc.;  efficiency  of  manufacturing  appliances  and  processes  for  the 
working  of  metals;  suitable  specifications  for  metals. 

(d)  CHEMICAL  METALLURGY. — Small-scale  preparation  of  pure 
metals  and  alloys;  determination  of  effect  of  metallurgical  auxiliary 
materials,    such    as    slags,    deoxidizers,    and    refractories,    upon 
properties  of  metals  prepared ;  determination  of  gases  in  metals. 

(e)  MELTING  OF  METALS. — Determination  of  the  melting  tem- 
perature (or  temperature  range)  for  any  alloy  or  metal;  methods 
of  casting  ferrous  and  nonferrous  alloys;  methods  of  molding; 
properties  of  molding  sands;  preparation  of  alloys  to  order;  and 
furnace  operation  as  applied  to  the  melting  of  metals. 

In  some  cases  properties  not  included  in  the  above  list  may  be 
necessary,  such  as  temperature  coefficient  of  electrical  resistance 
or  of  thermal  expansion,  specific  heat,  etc.  Such  must  be 
arranged  for  in  advance  by  correspondence ;  likewise  examinations 
of  a  very  special  character,  such  as  the  radiography  of  metals 
and  the  determination  of  the  magnetic  characteristics  of  iron 
and  steel. 

For  further  information  concerning  the  tests  of  a  metallo 
graphic  nature  carried  out  by  the  Bureau  of  Standards,  Circular 
No.  42,  Metallographic  Testing,  should  be  consulted. 


THE  LIBRARY  •* 

UNIVERSITY  OF  CALIFORNIA 
T/>R  ANGELES 


6?0 


at.  I   Ban  of  standards  - 
Structure  and  related  properties  of 

UNIVERSITY  OF  CALIFORNIA  LIBRARY  metals, 

Los  Angeles 
This  book  is  DUE  on  the  last  date  stamped  below. 


OCT  2 8  1968 


Form  L9-10m-10,'56(C2477s4)444 


